There aren't very many.
There is Countee Cullen, the Harlem Renaissance poet, but Cullen wasn't his birth name (see below).
There was Bill Cullen, the game show host. My great-aunts thought he might be a cousin but I don't think so. But maybe that means that their uncle Joseph had a son named William or Bill, we have lost track of his children.
There is Lord Cullen, who is/was the highest judge of Scotland.
There is Dr. Heidi Cullen on the Weather Channel.
There is Charles Cullen, the nurse/serial killer in New Jersey.
Here are some more suggested by readers of this blog:
Alice (McLaughlin) Cullen, of Glasgow, a leader of the local Labor Party and the first female Roman Catholic member of the British Parliament.
Bill Cullen, successful businessman (Renault Ireland), author and philanthropist.
Bernard Cullen, author and Professor of Philosophy at Queen's University, Belfast. His biography (scroll down)