Father Jack Hackett, played by Frank Kelly, is an elderly, decrepit, foul-mouthed, lecherous, alcoholic priest who frequently lapses into violent behaviour, particularly when his fellow clergy deny him drink or try removing him from his armchair, where he is usually found sleeping (or, more likely, passed out). Left irrevocably damaged by his life of sloth, lechery and alcoholic abuse, he rarely speaks in a coherent manner, instead expressing himself through a series of random shouted words (i.e., "FECK!", "ARSE!", "DRINK!" and "GIRLS!"). He regards Father Ted Crilly, Father Dougal McGuire and Bishop Brennan with undisguised contempt, although he seems to be more tolerant of Mrs Doyle; possibly because she has a fond maternal affection for him and frequently lavishes him with drink. However, when she once offered him a cup of tea, he was outraged and hurled the cup at her head. He is hinted as being mentally defective as shown when Ted is able to calm him by showing him the colour blue or puts him to sleep by placing a box on his head. Despite this, he has been said to have executed careful plans such as hitting Ted with a car or placing a spider in his mouth. Ted often treats him with an overgenerous level of respect, possibly because of his length of service.
Jack is extremely violent, frequently lashing out at all and sundry. He apparently once kicked Dougal up the arse and also once even punched Bishop Brennan in the face, but most of his violence is reserved for Ted, who explains that 'whenever he'd hit you or whatever, he'd never do it out of spite. He'd do it because he thought it was funny...!'
During the episode "Grant Unto Him Eternal Rest", flashbacks of Father Jack's earlier life as an active (and sober) priest showed him to be a highly conservative fire and brimstone-style Catholic School preacher who enjoyed tormenting students with threats of eternal damnation and giving out heavy amounts of corporal punishment. Another flashback implies that he had a less-than-healthy interest in the female students. Dougal also points out that Jack was the first priest to denounce The Beatles because "he could see what they were up to". Jack is held in very high regard by the other elderly priests, with one even exclaiming at his (premature) wake that Jack should have been made Pope rather than John Paul II.
While it is never explicitly stated why Bishop Brennan has doomed Jack to Craggy Island, it is mentioned in "The Passion of St Tibulus" that it has something to do with a wedding ceremony he performed in Athlone. In the episode "Tentacles of Doom", with the promise of more "drink", Ted was able to train Jack to say "That would be an ecumenical matter!" and "Yes!" so that he would be able to convincingly circumnavigate any questions put to him by a party of visiting bishops. Jack also has a fear of nuns, whether inebriated or not. He hates doing confession - partly attributed to his fear of being buried alive - and regards the sick and poor with contempt, referring to the needy as a "shower of bastards". It is later revealed that he has saved up £500,000 his will which Ted partly attributes to "never giving money to charity" and the fact that "he wouldn't wear trousers during the summer".
A recurring theme in the series, Jack's alcoholism has almost rendered him a human vegetable (although he had the presence of mind to find a bag containing an incriminating tape of Bishop Len Brennan when the latter threatened posting Jack, Dougal and Ted to places even less desirable than Craggy Island in "The Passion of Saint Tibulus"). He is constantly intoxicated and therefore permanently unaware of the people around him, except when they interfere with his daily routine of sleep and drink. Jack's drinking is not limited to alcohol, and he will drink almost any liquid he can get his hands on. Sometimes these strange choices of drink will have equally strange effects on Jack; Toilet Duck for instance causes him to experience hallucinations, and floor polish makes him temporarily appear to be dead, to the extent that he apparently actually started to decompose. However, his alcoholism has also left him with the incredible ability to tell what vintage a bottle of wine is simply from the sound of the bottles clinking together ("The Old Grey Whistle Theft").
In the episode "Cigarettes and Alcohol and Rollerblading", Ted observes that Jack hasn't been properly sober for twelve years, and when he does finally leave his inebriated state for a moment, realises to his horror that he's still on "that feckin' island" (which might explain why he turned to drink in the first place). His alcoholism makes it apparent that he has little memory for the names of household objects, as he picks up a spoon and demands to know of Ted, "what the hell is this?"
The drink has left Jack with scant regard for his personal hygiene; he is always depicted with a ring of scabs around his mouth, an unkempt stubble and stains down his clerical collar and smock. In "Tentacles of Doom" he is given a shave of his hair, but by the end of the episode it is back. In the episode "Night of the Nearly Dead", Eoin McLove points at Father Jack and claims "This one here, this one smells of wee". In the final episode, "Going to America", Ted imagines Jack as a loveable, grandfatherly figure, seated in a rocking chair and dressed in a cardigan rather than his usual dark garb. He is also notably cleaner-looking, without the scabs around his mouth, or his milky eye.