Completely randomly, but still Cullen related:
I just watched a true crime documentary, and one of the women they were interviewing had the surname Cullen. So it can be a given name or a surname. 
I also found this origin which I thought was rather neat:
Origin of the name Cullen:
Borrowed from the Irish, Cullen is the Anglicized form of the Irish surname Mac Cuilinn (son of Cuileann). Cuileann is an Irish name meaning “holly.”
Edit: Ignore my dumbarsery and stating the obvious. Got caught up with "Oh hey, she has the last name Cullen!" orz