Cillian McGrath (キリアン・マクグラス Kirian Makugurasu) was the second-in-command of the Advanced Irish Republican Army, a splinter group of the Provisional IRA. Having engaged in pro-Irish reunification violence against the British state for over a decade, he gradually became exhausted due to losing so many comrades and, as a result, grew resentful toward his faction leader Michael Connolly.

Cillian was an Irish man with a skinny build, pale skin, and short, messy dark hair styled with a widow's peak. He wore a dark-colored suit and pants, a light-colored dress shirt, and black shoes. He carried a semi-automatic pistol for his primary weapon and also concealed a small sleeve gun in the right arm of his suit jacket.

Adept at both hand-to-hand combat and using firearms, Cillian was a cunning fighter. Although he was a hardline Irish nationalist, he cared a lot about his comrades to the point that the continuous deaths of Advanced IRA extremists took a toll on his conviction in seeing a united Ireland. He himself was not afraid of being killed despite his feelings of anguish at seeing his friends die at young ages and in great numbers.

Nothing was known about Cillian's life before his later activity in the Advanced Irish Republican Army except that he had been involved in Irish republican terrorism for a dozen years. Despite his work and being a notorious person of interest to the British police, he also lived a regular civilian life in a neighborhood in the British Isles. Cillian lived with his dog Quinn, whom he frequently had his neighbor Abbott take care of while he was away on "business trips" that were actually Advanced IRA missions. His fellow IRA member Ian also hung out with him at his domicile.

Some time after the Advanced IRA had trouble procuring weapons due to Gaddafi refraining from sending more weapons to IRA factions via a North African route, the group needed to acquire weapons elsewhere. Accordingly, Cillian was sent to the United States to complete a negotiation with American gun runners to acquire Stinger missiles and other arms. On the East Coast, Ian accompanied him to the deal site, which was quickly raided by ATF agents due to an insider among the Americans. Though Cillian managed to survive and escape unharmed by fleeing on a motorboat, he was shocked that Ian had been fatally shot in the abdomen. He returned to the United Kingdom, having successfully secured the delivery of the weapons. However, he was greatly depressed after Ian's death, which served as the culmination of his exhaustion at fighting for the Advanced IRA, having lost other comrades across the years.

At a hideout, Cillian spoke with Michael Connolly one-on-one and claimed that there seemed to be more younger people in their group. Not ecstatic about completing his mission, the second-in-command told his boss about Ian's death, prompting Michael to offer brief condolences before telling him about news of an American agent coming to the U.K. to retrieve the Stinger. Admitting he was unsure if the American was part of the ATF, FBI or CIA, he assured his subordinate that their main obstacle was the London police.

Sighing, Cillian rhetorically asked how many more young men he would send to their deaths. Infuriated at the question, Michael could not believe he would ask that and shouted that they would never stop fighting no matter how much blood was spilled until they saw the day of a united Ireland. Calming down, he claimed to Cillian he understood his sadness about Ian and added that the deaths of their brothers and sisters could not be in vain. Shouting that they would drive the Brits out of Ireland, he quieted down again and advised Cillian to go home and let his dog Quinn comfort him, so the latter left the office without saying a word.

At home, Cillian walked his dog in his neighborhood and returned home to feed the dog. Speaking to his pet, he asked the animal how it was feeling, remarking that Ms. Abbott and her daughter really seemed to like the dog. Relaxing with Quinn on his couch, Cillian informed his pet that Ian died, jokingly recalling that it was really hard to feed the dog after Ian once bought fancy dog food for him. Seeing his dog look at him, the disheartened man admitted that he missed Ian too. He then used his telephone and placed a call, requesting to speak to Michael for further instructions for their next step in the grand plan to carry out a terrorist attack against the British government. Subsequently, Chester Shankly agreed to lure the American agent and her Met associates Jerry Smith and Emily Thompson into a trap. At a pub in Watford, Cillian and his men waited outside so that they could ambush the officers as Shankly fled. As soon as they saw Eda and the two officers run out, the Advanced IRA extremists immediately opened fire. As Emily and Jerry fended off some of the fighters, Cillian ran away from the van he used as cover and entered the pub to ambush Eda from the backdoor.

She heard the door open and turned around, instinctively charging her shoulder into his abdomen before he could fire. The two began to struggle and missed their first shots. Eda took aim at him after she pushed his arm away, but Cillian kicked her pistol out of her hand. Countering again, she grabbed his exposed foot and pushed it upward to upset his balance, punching him in the face. Cillian dodged Eda's second punch and backed away. They smirked and commended each other's endurance, but they were interrupted by Emily ordering Cillian to surrender. Refusing to do so, the second-in-command bid goodbye to the CIA agent and ran to a fence. Though Emily took aim at the extremist, Eda blocked her line of sight because she needed him alive.

Afterward, Cillian returned to one of the Advanced IRA's hideouts where Michael was finishing a call concerning their weapons shipment. Walking into the office, Cillian complained that his mission was essentially acting as a decoy to lure out the police. Ignoring the complaint, Michael said he was disappointed that Cillian could not kill the agents, especially the American among them. Apologizing for it, Cillian reasoned that she was tougher than expected, but Michael responded it did not matter since they needed to plan their next move. That night, the right-hand man felt he had nothing to lose due to losing so many comrades and decided he would die fighting the next time he encountered the police. Cillian thus handed over his dog to his neighbor Abbott, who falsely believed she was watching the pet since he was going on another "business trip." Additionally, he discreetly met up with his comrade Morton and ordered him to intentionally tip off the authorities about the Advanced IRA hideout from where they planned to launch the missiles at the British air force base.

The next day, Morton bombed a car outside New Scotland Yard as Eda and her group exited the headquarters, running away before being shot in the leg. He quickly told Eda the location of the next terrorist attack while Cillian, his boss and fellow Advanced IRA subordinates were picked up by a delivery van to travel to the missile launch location, which was in a small neighborhood. Unknown to the Advanced IRA, Morton's leak allowed for the air base to halt all takeoffs and landings as well as giving the police and military time to mobilize against a potential attack. Eda also drove Jerry and Emily to the hideout located a few kilometers south of the air base. As they headed there, Cillian loaded his firearms as Michael confessed that he was proud of the former for quickly recovering from his sadness about Ian's death. Practicing his aim, Cillian smiled and asked if he really thought he had changed. When one of the extremists noticed an unmarked police van driving up to the hideout, the Advanced IRA members prepared for battle.

As soon as Jerry knocked on the front door, the extremists opened heavy fire on the officers. Going to the back, Eda sneaked into the back door and turned around the staircase corner when she and Cillian saw each other. The latter shot at her, forcing her to take cover as he urged Michael to escape through the window. When Eda shot back, Cillian laid on his stomach as Michael jumped out the window and onto the ground, being spotted by the two officers as he began fleeing. Whereas Emily engaged in a shootout with Michael and soon defeated him, the exhausted Eda assessed that she could question Cillian about the Stinger knowing that he led the arms trafficking deal in the United States. While waiting for an opportunity to attack, to Cillian's shock, an elderly extremist pointed his Stinger launcher at the CIA agent. The man asked what he was doing, to which the senior declared that they would not go down without using the missile that took so much work to acquire. Eda jumped to the ground as the man fired the missile, but to their surprise, the Stinger missile turned out to be a dud and merely fell down the stairs onto the ground without exploding or deploying its flight rotor.

Improvising, the extremist pulled out a grenade, but Eda shot him a second later before he could throw it, instantly killing him. Running down the stairs from the live grenade, Cillian jumped over the stair railing and reloaded his rifle, dodging Eda's shot. As he turned around, his rifle barrel got stuck between two stair balusters when Eda kicked it downward, mocking him for using his large gun in such a small place. She pinned him against the wall at gunpoint and rhetorically asked if he deliberately told the Advanced IRA car bomber where he was. Eda demanded to know where he got the Stinger from, but he only answered that such a question would do her no good. Irked, she threatened to shoot him full of bullets if he did not talk. Holding onto her pistol and pulling the barrel to his forehead, Cillian grinned and encouraged her to kill him, taunting her for not following through on her threat.

He took her off guard by attempting to pull the trigger, causing her to point the pistol away as the shot missed him. Cillian grabbed her neck and tried pushing her downward, but Eda pushed her foot against him and flipped him over her using his weight. Eda frantically crawled for her pistol as Cillian tried pulling his own pistol from his coat. As they aimed at each other, they both shot at the same time, but Cillian missed and was shot in the abdomen, admitting he underestimated Eda for intentionally missing his head. Frustrated, she yelled that she would have gladly killed him if she did not have orders to follow, ordering him to fess up. Claiming he still had the advantage, the downcast Cillian elaborated that he had seen many of his comrades slain in the dozen or so years he had been fighting. He professed that his peers became younger as time passed, mentioning that all of their deaths haunted him. Annoyed even more, Eda retorted that she did not come to the U.K. to hear his whining. Cillian reminded her that she came to get information from him, rhetorically stating that he won if she could not get it out of him. Suddenly, he deployed a sleeve gun on his right arm and pointed the pistol under his jaw. Giving a final smirk, Cillian asserted that he and his comrades would win in the present and the future, then killing himself. With the deaths of the highest-ranking Advanced IRA members as well as the defeat and capture of other extremists, the group had effectively ceased to exist.

After the police operation which foiled the air base attack, no one outside of those involved knew about the deaths of Cillian McGrath and Michael Connolly. Although the Advanced IRA was effectively no more, their gunrunning allies in the United States, particularly the Fenians, were still at large. Chloe Reddington, Michael's wife, did not learn about her husband's murder. Exploiting this fact, the CIA had Eda pose as Chloe in order to speak with an intermediary who would give her the details about the next arms smuggling deal which was to take place in the United States. Additionally, the ATF and NYPD officers who had operated together to stop both the Advanced IRA and the Fenians were not informed of Cillian's death. In particular, Dent, who had previously spotted Cillian during his participation in the ATF raid that killed Ian, was lied to by Eda about the Advanced IRA and Cillian still being active during the CIA's joint operation with the ATF.