The actor was accused of being a child molester and likened to Jimmy Savile in Twitter exchanges last week.
He has referred the comments to his solicitor to see if the people behind the accounts can be identified, with the possibility of making a complaint to gardaí.
“I’ve never heard anything like it before. It was so vicious and so quick. It’s like a kick in the stomach. I just couldn’t get over it,” the 52-year-old told Katie Hannon on RTÉ Radio One.
One Twitter user called the actor a “paedophile fool” and said there were “shades of jimmy saville” [sic] about him. Another alluded to his charity work at hospitals and suggested it gave them shivers to have “celebs hanging around vulnerable kids”.
Despite the allegations and innuendo, Mr Cowan plans to stay on the social media platform. But the experience has prompted him to think twice about engaging with other Twitter users.
“I loved Twitter and I loved engaging with people, especially the fans... I’ll certainly think very carefully about responding to something else, I won’t be engaging as much with people anymore,” he said.
“It should be just a fun thing to do, to just engage with people. There should be no problem with it, but unfortunately there are people out there and they have attacked me on it.”
He said the accusations were vile and without basis.
“I genuinely didn’t think people could be so vicious.”
Mr Cowan plays the son of Mrs Brown, played by Brendan O’Carroll, in the smash BBC TV comedy.
The Christmas special edition of the show was the most watched show on Irish TV, with an average 607,500 viewers tuning to RTÉ One on Christmas Day.
Daniel O’Donnell proved a hit on the same channel on Christmas Eve, when his festive special attracted an average audience of 342,600, while his Christmas Message with wife Majella attracted a total of 97,100 viewers next day on TV3.
Old classics remain firm favourites, with Father Ted, Harry Potter, Willy Wonka & The Chocolate Factory, The Wizard of Oz and The Commitments mixing it with soaps and new movies.
Fair City’s Christmas night episode had a 423,700 average audience tuned in to RTÉ One, with TV3’s Coronation Street and Emmerdale drawing averages of 234,400 and 195,700, respectively, on the same night.