The Dubliner's show, which mainly features members of his family, was the second most watched in Britain on St Stephen's Day.
The only other programme to attract more attention was sitcom Miranda. It was watched by 9.47m on December 26, just ahead of Mrs Brown, who had close to nine million viewers.
EastEnders again won the battle of the soaps with 8.2m, while Coronation Street had 7.8m.
Others winners were Doctor Who which got 7.58m viewers, Strictly Come Dancing (7.76m) and the Christmas Day episode of The Royle Family (7.68m).
Mrs Brown's Boys was broadcast on Christmas Eve and Christmas Day on RTE but the Irish station has yet to release its viewership figures.
It has been a surprise success on the BBC, having already won a Bafta for the O'Carroll family.
Brendan O'Carroll's real-life wife plays his TV daughter, his TV son's wife is acted by his daughter, and his big sister is his neighbour and best friend in his sitcom.
"We are not the Waltons, that's for sure," he said recently in an interview.
The new series will also introduce household favourites Joe Duffy and Derek Mooney to British audiences.
O'Carroll explained how, in the opening episode, Mooney plays a hypnotist who takes control of Mrs Brown's mind but forgets to "unhypnotise" her.
"He does a really good job and he's really slick," he said.
"A couple of episodes later, we've a psychic. Rory (Rory Cowan) and Kathy (Jennifer Gibney) decide to get a psychic into the house so that Agnes can talk to (her late husband) Redser.
"And that psychic is played by Joe Duffy, who is mental in it! He really goes for it. He scared the s*** out of me," O'Carroll said.