Colm Meaney and Tina Kellegher are just 14 years apart.
In the scene where Dessie celebrates the birth of his grandchild by ordering a pint of Guinness and downing it one go, Colm Meaney's post-Guinness smile of achievement was his own reaction to successfully pulling that off.
The book was a sequel to The Commitments but the characters couldn't be used here for copyright reasons so were all replaced. Same thing happened with the third book, The Van but Colm Meaney played the father in all three movies.