Guitar because I was a kid and everybody and his dog played guitar (and still does). Good for accompaniment.
Mandolin because once I'd heard Barney McKenna, John Sheahan, Andy Irvine, Bill Monroe, Sam Bush and Tim O'Brien I was hooked. Also because it is such a great little instrument. Light, portable, and the common folk, old time and bluegrass major scale shapes just fall so conveniently under the fingers.
Whistle because of John Sheahan, Micho Russell, Paddy Moloney and Liam O'Flynn. They're so cheap and portable why the hell would anyone not play one (Same goes for harmonica)?
Fiddle because it seemed natural to try it after mandolin, and it's a wonderful instrument. Shame I'm not a wonderful player.
Bodhran to annoy people.