Confirming the good news, the RTE star said they were "over the moon" about the prospect of adding to their family and that they already know it's going to be a little girl.
He and his other half already have a son, Dashiel, and are thrilled at the thought of a new addition to the clan.
"Dashiel is five now and as far as he's concerned, this was all his idea," Mario explained.
"We already know that it's a girl so we have the room all painted pink in preparation. Blathnaid doesn't like surprises so we decided to find out the sex of the baby.
"She's due the first week in February and everything's going well."
The impersonator (42), who's receiving rave reviews for his RTE Monday night show, became a father for the first time when he was 37 years old and said it sparked a massive cutback in his social habits.
He said he was just like so many other Irish people who enjoyed their pints at the weekend, but having a baby changed all that.
"I had a kid when I was 37 and that resets your perspective entirely. I have been lucky that I have been allowed to go off on tangents for a living."
The star is also flying high in his professional life, given the massive ratings he pulled in for the Mario Rosenstock show.
It sees him parodying everyone from An Taoiseach Enda Kenny to Miriam O'Callaghan and Roy Keane.
RTE are so thrilled with its success that they have already commissioned a second series of the TV show.