The In Bruges actor played and chatted with children for several hours on Christmas Day and also brought them presents.
And it wasn't his first festive visit -- he has carried out similarly unheralded visits to Crumlin for the past few years.
"He keeps a low profile when he comes in. He doesn't want photographers or anything," a well-placed source told the Herald about the actor's visits.
"He comes in every Christmas Day and spends a lot of time with the children.
"He has bought Nintendo DS and Wiis for them and plays with them for a couple of hours," the source added.
"He's really popular with the staff and really genuine, and the kids really love and appreciate his visit."
The actor, who recently became a father for the second time to baby son Henry, does not want to court any publicity when visiting patients.
And back in 2003, Colin (33) helped support the Special Olympics which was hosted then in Ireland.
Since then, he has been a tireless campaigner for the cause, regularly giving up any free time he has to help the organisation with their activities and further raise their profile.
Much of Colin's spare time is spent in Los Angeles, so he can be with with his six-year-old son James, who suffers from the rare, neuro-genetic condition, Angelman Syndrome.
Colin often credits little James, who lives with his model mum Kim Bordenave, with saving his life thanks to the positive influence he has held over his father since he was born.
The award-winning actor spent Christmas at home this year, and brought elder son James (6) to enjoy festivities in their father's native area of Castleknock. Baby Henry, who was born in October, is also believed to have made the trip.
A source close to the former hell-raiser has confirmed how the actor will be jetting home with his girlfriend Alicja Bachleda (26) and their tiny tot on Christmas Eve.
"Colin's a real family-orientated guy and nearly always makes it back for Christmas anyway, no matter how busy he is with his filming schedule, but this year it will be extra-special thanks to his new baby."
His Polish girlfriend gave birth to their first baby on October 7, but managed to keep the happy news under wraps from the media for nearly a week before the actress's brother spilled the beans.
The pair met while filming Ondine in Cork and have been an item since.
On Sunday, Colin attended the belated wedding reception for his brother Eamonn, who wed long-term partner Stephen Mannion in Canada.
The party was held in Krystle and Colin attended the event with son James.