Brendan Gleeson plays Martin Cahill, in his most prominent leading performance before his work with the McDonagh brothers, and from the get go Gleeson benefits from having a fairly close physical resemblance to the real Cahill. The reality of Cahill though doesn't come too deeply into the film itself which takes the approach of making him somewhat of a folk hero of sorts, since in reality it appears Cahill was an even more devious sort than the film presents him to be. Gleeson though is terrific in creating the "heroic" version of Cahill that we meet early in the film as he does things his own way, and that's too an even more absurd extent than most individualists. Cahill is not a usual crime boss, first of all since he focuses almost exclusively on robberies, but also takes rather absurd and flamboyant method to crimes. Cahill almost doesn't hide his crimes to the police instead he carefully hides any possible evidence that could tie them to him.
Gleeson is perfect for the role, and that's not just because of his physical likeness either. Gleeson is very interesting in the way he can have quite the commanding presence while bringing such a relaxed charm in his manner. That's certainly his approach for Cahill which is an effective one to be sure. Gleeson's charm as well makes Cahill quite the likable character here despite the fact that he's a criminal who only commits crimes for personal benefits, and is actually not at all opposed to violent crimes. Gleeson though portrays such a joy in Cahill whenever he commits a crime in his very peculiar and strange ways, such as one where he deposits money at bank then hangs out at the police station for alibi while his own men steal the same exact money right back from the bank, that it's difficult to not kinda have the fun right along with him. Although Cahill is technically quite smug and overconfident in terms of his actions, Gleeson's performance is able to smooth over all of this due to just how accessible Gleeson makes Cahill.
Cahill is a weird crime boss, not only because he stays in plain sight, other than his habit of always covering his face in public, but also because of his fairly informal way of dealing with his men. Gleeson is quite interesting as he makes Cahill odd way of things rather believable because of his performance. There is a great charisma about Gleeson making it easy to see how Cahill is so easily in charge of such a large group even though the structures involving his game are pretty thin. Gleeson has that command even though there is such an ease in his performance at the same time. Of course Cahill on occasion does dish out some punishment, such as when he crucifies a guy to a pool table to ensure he's telling the truth. Gleeson is appropriately cruel in the scene effectively revealing the darker edge to Cahill. Gleeson manages to bounce back from this naturally though when Cahill takes the same man to the hospital himself to ensure that the man recovers from his wounds.
Everything is not fun and games for Cahill because despite his considerable success with his absurd crimes Cahill can't keep up the streak. One of the reasons Cahill can't keep the streak going is because of the streak makes it so the cops take him so seriously that they tail him constantly with police always watching his house. Gleeson is terrific in carrying himself in general with that rebellious attitude and confidence of Cahill and then proceeding to wear it away as his life basically starts to become to much for him. There are the occasional moments where something goes wrong Cahill and Gleeson is great by showing Cahill lose his resolve for a brief instance. In the instance Gleeson suggests the real vulnerability of the man in certain circumstances such as when he has to get extremely abrupt with the I.R.A. since the organization wants in on Cahill's earnings. Gleeson handles these well by showing Cahill's unable to keep his personal going under greater pressure.
Throughout the third act of the film one things after another goes wrong for Cahill as he makes some poor decisions, his underlings begin to threaten his freedom, he develops diabetes, and of course the police never stop hounding him. Gleeson is very good here as he portrays Cahill keeping his brave face of joking and messing with the police but with moments of losing his cool which become more and more intense. Soon though it becomes clear that everything is becoming to much and Gleeson is most moving when he presents Cahill even losing his ability to always smile to the police. One of his best scenes is when Gleeson shows Cahill losing almost all his reserve and Gleeson portrays it as Cahill completely breaking down but constantly trying to keep himself together though he simply is not able to. Gleeson gives a great portrait of Cahill's emotional rise and fall as he shows that he is basically finished mentally even before he receives his fatal wounds from the I.R.A.