Wicklow getting the most votes here I think!
OP, if you have two, max three days here, the closer you are to Dublin the better for your explorations. You can always return and do another route when you have more time.
Wicklow is about an hour from the City Centre max, good motorway links on the M11, then you branch off to the country roads which are very picturesque.
Check out...
Blessington lakes,
Glendalough,
Greystones,
Roundwood,
Avoca,
Powerscourt Waterfall and gardens,
Mount Usher Gardens, and so much more. Can be done in a day or two at leisure.
I understand that there are so many other beautiful places all around Ireland, but for such a short trip I don't think you could go wrong with Wicklow. I have no connections with the place, but it has everything, and is stunning. (Unless it's pouring down with rain!)
Hope you have a lovely time whatever you decide.