A lot of military badges and buttons here, some from Irish regiments, which feature a harp.
13thcolonyapparel.com

More harps here:
www.threefeatherspewter.com

Not much luck on buttons with a shamrock on them, although there is a four sided Celtic knot design that's sometimes referred to as a four leaf clover, even though it doesn't look like a clover.

No harps at this site, but a good selection of Celtic Buttons
http://www.faireware.com/button1.html