Well you can certainly get to Letterkenny (at least you could the last time I went - 10 years ago) - coach leaves from outside the gallery at the top of O'Connell St. Sorry can't remember name of operator but it was something very Donegal IIRC - McGonagles maybe?
Caveat - you are probably thinking of McGinleys bus thou - www.johnmcginley.com - which goes from Parnell Sq (near Chapter One) through Derry and on to Buncrana & Carn