Brian, has long been among the most popular of the Irish imports for boys. It landed on the US popularity list in 1925, entered the Top 100 where it remained for 60+ years, reaching the Top 10 in the 1970s. Now, though it has gradually been replaced by such upstart brothers as Aidan and Liam, Brian still manages to hang in at a respectable Number 150.
In Ireland its claim to fame was tied to Brian Boru, the most famous of all Irish warrior-kings, credited with driving the Vikings out of Ireland. In the modern world, Brian has some pretty diverse associations--to newscaster Williams, the movie Monty Python's Life of Brian and as Marilyn Manson's real name.
Bryan is a common alternative spelling.