Not that I am any expert on this whatsoever, but I do have a cousin who lives in Northern Ireland way up by Portrush, and I've asked him about just this from time to time.
His answer is what you'd expect: that is, the weather can be just fine in March, and might even be better than summer... but also can be brutally cold and wet.
That's really the standard answer for any day of the year in Ireland, God love it.
So... the answer here is it's not like you're guaranteed good weather even if you go in the summer! Any trip over there always has its risks.
So don't let weather stop you if you want to go in March. It could be great and if it's not, well then that's part of the learning experience.
TH
ps - if you do go, let me know... I've mentioned it on here many times but my cousin is a member at a great club called Castlerock and I'm sure he could set up matches against local highschoolers, or club members, if you wish... talk about a life experience!