I've read a few things online that act like most X chromsome DNA is Celtic and most Y chromosome DNA is Nordic, but it sounds like bullsh*t.
I believe there are more men of Celtic descent that any other, aside from maybe Genghis Khan.
Is R1b the most dominant?
What is Celtic X haplogroups?
Is the Celtic Y chromsome R1b more dominant that any other?
You could say R1b-P312 and most of it's sub-clades, in general.
OConnor is right, you need talk specifics, as the Celts weren't/aren't one homogenous group, except in regards that they usually carry high levels of R1b-M269 and below.