What to see From Galway to Dublin?

Clonmacnoise, just outside of Athlone.

Second vote for Clonmacnoise. Also, consider stopping at Tullamore for the D.E.W. Heritage Site (whiskey tour and samples), we thought it was pretty neat (although I really love whiskey), and Charleville Castle is right nearby as well. You'd definitely have time for all three over the course of a day.

Another for Clonmacnoise. Here's some other possibiliites, some may ionvolve a detour, but if you get an early start they are possible:
Hill of Tara
Newgrange and/or Knowth
Monasterboice High Crosses
Trim Castle
Rathcroghan

Thank you for the suggestions. We will definitely do Clonmacnoise and check out the others. We already planned on hitting Tara and Bru na Boinne, hopefully we could add Monasterboice to that day. It will be up to my husband if we hit Tullamore. He is the whiskey drinker. The help is much appreciated.

Depending on what part of the year you will be traveling, it might be difficult to get into Bru na Boinne unless you get there early in the day. In peak tourist season, tour buses often take large numbers of the available shuttle seats and individual visitors might not get to go in. I went there in February and had to wait through two shuttle times before I could go (I arrived around 1:00 pm). I don't think you will be able to fit all of those locations into one day.