Help me find a clip of an actor ad-libbing Fairytale of New York on a chat show

I must have both the actor and the show wrong — I truly thought it was Chris O’Dowd ad-libbing Fairytale of New York a cappella on Graham Norton from his seat

NOT CONFUSING IT WITH HIS ‘O DANNY BOY’ ON ROB BRYDON — that’s the clip that REMINDED me in the one I’m seeking (+ I’m fairly sure the actor singing was sharing the stage with other guests.)

I’d love some help because even stripping the search down to bare variables only turns up planned, musically accompanied performances on various chat shows. I swear I saw this brief, unexpected rendition (by whoever it was, on whichever show it was) being warmly received by the audience!