Although now lecherous and violent, legend has it that Jack was once a famous priest in the local area, greatly admired and respected in the community (however, some handy flashbacks reveal that Jack has always been lecherous and violent, though, at one point, apparently quite handy at football as well).
But Jack is in decline. According to Ted, his love of alcohol and violence has reached the Stage Two, a dangerous condition affecting ageing priests that is stripping away his humanity.
Jack is frequently referred to as an animal and often treated like one too. At one point he escapes from the parochial house and Ted and Dougal enlist the help of a tranquilizer gun to bring him down - although they decide against using it, because Jack normally finds his own way home.
He also has an uncanny ability to impersonate Sir Bob Geldof, which he uses to acquire girls and booze. These, as his remarkably succinct vocabulary indicates, form the focal point of his limited mental faculties. He also has incredibly hairy hands.
Jack's preferred form of exit from the Parochial House is the living room's front window, and he has an irrational fear of nuns.