Since we have 2 nights now, we are figuring we would see the countryside on the first day and then stay in Dublin on the 2nd day. What should we not miss when we go to the countryside? I believe we would like to visit a castle, and perhaps some ruins?
Two countryside options within a few hours of Dublin are the monastic ruins at Glendolaugh (sp?) and the ancient burial mound at Newgrange.
Go to Newgrange, definitely. Knowth is also visited from the same visitors center, so you could do both at one time. Glendalough is wonderful, too, and only a short drive through the mountains south of Dublin. There are also lots of day trips available to Glendalough from Dublin by bus.
Dublin bus has a tour that goes to Newgrange and also the ruins of an abbey that I can not recall the name of. www.dublinbus.ie
If you want something different you could go to Maynooth - just outside of Dublin in Kildare - to visit the ruins of the FitzGerald castle there. Very important site historically and practically unknown yet to the foreign tourist trade. They now have a good interpretative centre right there at the site.
Newgrange is fine also but always very crowded and takes a long time to get to - they no longer allow direct access so you have to go to an off site centre and be bussed there - the line takes hours sometimes.
Ennis, the visitor center for Knowth and Newgrange is a five minute bus ride to the actual site. I don't know how crowds would be in May, but when I went there was no line at all (early March). There were about 20 people on my bus.
We just did the Newgrange site 20 July and had no lines. They were very organized with the turn-over of the tourists by bus. Also, for a castle that is relatively easy to access there is Malahide Castle. There are bus tours or you can just take the DART to Malahide and walk about 10 minutes to the castle.
Agree lines can be long at Newgrange. Visited May 29 and earliest tour was 3 1/2 hour wait. Well worth it, but plan to spend the day. The visitors center is very nice, and has a cafe.