[20 Dollar Nose Bleed]

Between your shoulder blades. I think that's a great place for guys to have tattoos.
The best thing is that you have an almost unlimited amount of room and can even add to it later on without problems.

If you don't like that idea then maybe your upper arm - but possition it so that the celtic knot goes down onto your forearm when you add it. That way it's not so centred.