It's a decent name. It feels like an older person's name to me, probably because the only Dermot I can think of atm is the late DermotGraham, who played Father Ted. I voted for the Diarmuid spelling, but Dermot is okay too.
My favorite is a tie between Diarmid and Diarmuid...is there a reason you prefer Diarmuid?Dermot seems to be the BtN favorite, but I don't know if I like it or not. It might remind me too much of Kermit.