Revealing mistake: When Jack's wheelchair is at the top of the cliff and about to fall off, it's obviously a stuntman and not Frank Kelly sitting in it. Similarly, when the wheelchair falls, the dummy used isn't even dressed like Jack. (00:14:25)
Continuity mistake: Father Dougal's hair changes position during the scene in the car, before they go on holiday.
Revealing mistake: When Ted and Dougal are climbing the big rock after seeing St. Kevin's Stump, it's so obvious that a light is being shone on them from behind. They both have shadows even though it's a dark and cloudy day.
Chosen answer: It's characteristic of someone putting on a fake Irish accent.