Brendan Gleeson and John Michael McDonagh came up with the character of Father James at the end of their collaboration on The Guard (2011). It was tailored specifically for Brendan Gleeson, who in turn helped to develop elements of the character.
The painting that the character of Michael Fitzgerald (Dylan Moran) urinates on is The Ambassadors by Holbein. Aside from the unique skull that can be seen in the bottom center of it, further examination of the painting shall reveal a crucified Jesus on the top left corner, hiding behind the "curtain".
Father Lavelle says "God is great and the limits of his mercy have not been set" This is a quote from Jean Amery's book "On suicide: A discourse on voluntary death"
Father James Lavelle: I've always felt there's something inherently psychopathic about joining the army in peacetime. As far as I'm concerned, people join the army to find out what its like to kill someone. I hardly think that's an inclination that should be encouraged in modern society, do you? Jesus Christ didn't think so, either. And the commandment "Thou shalt not kill" does not have an asterisk beside it, referring you to the bottom of the page where you find a list of instances where it's okay to kill people.
At the Q and A after the premier in Sydney, it was asked of McDonagh who killed Bruno and he gave the hint that the killer was in the bar and had a bandage on his hand/arm, but he didn't reveal who it was. Brendan Lynch the bar man has a skin coloured plaster on the back of his right hand.Another possibility could be Milo Herlihy (Killian Scott), to prove he could kill and so be fit material for the army.Another possibility is his daughter: She was the only one who had access to his house and was also wearing a bandage. This would also explain the last scene.