Hi there,
I will be doing several days of hiking in Ireland in August. Here are some ideas that might work for you, that I've run across:
-If you're thinking of visitng Glendalough Monastic City while in Dublin there some loop trails in the area that might suit your needs: http://www.wicklowmountainsnationalpark.ie/WalkingTrails.html
-Also the Wicklow Way starts at Marlay Park on the outskirts of Dublin, you could walk a portion of the trail from there as well: http://www.wicklowway.com/
-The Howth Peninsula has several short loop day hikes that would be easily accessible from Dublin (click on the #2 Dublin for the Howth walks): http://www.discoverireland.ie/Walking/looped-walks.aspx
I've personally found the Lonely Planet Walking Ireland book a great resource with hikes listed all over the country. Also great resources are the Discover Ireland website referenced above and the company Over The Top Tours (http://www.overthetoptours.com/) also offers day hikes.
Hope this gives you some guidance and you're able to explore the Irish trails!
ps. Forgot about the Limerick area! This has been the best website I've found so far for day hike (or longer) trails in the Shannon Region: http://www.shannonregiontrails.ie/FindaTrail/Walking/