Siobhan Fallon (pronounced "sha.vaun") is an actress I see everywhere in supporting roles. I'll be watching a sitcom or a dark drama and there she will be.
What finally got me to look up her name and read a little bit about her were the two most recent rolls I've seen her in. The first was in the sitcom 30 Rock (credit: Siobhan Fallon Hogan) as one of Jack Donaghy's emotional, and very funny, Irish catholic sisters. The next time I saw her, she was a prison guard, in the movie Dancer in the Dark, who befriends a young mother on death row.
She is a big personality on the screen; in part due to her New York accent. At the same time she never over shadows the scenes she is in or the other actors. I guess that is called talent.