Eric,
I have no good idea if there are 'must see' Irish lighthouses. I think that there over 80 to choose from.
In our traveling days, any trip that included a boat ride was considered a good trip. So I'd be looking for a least one ferry ride to an offshore lighthouse. Arranmore is one.
What I can offer you is to allow yourself time to really enjoy the ones you do stop to see.
Have a great trip!
Rick