Here's how it works in my brain:
You take a man of average attractiveness, add a guitar and an accent from the British Isles (in this case, Northern Ireland), and wham! I find him hot.
And it's not set in stone. You can substitute drums and make the guy be from Sussex. Add gorgeous blue eyes, a couple of cats and WALLOP! I'm in total lust.
The fact that both of these men can actually sing? Icing on the cake.