re: 3 days in a Ireland

Don't bother renting a car.

With just 3 days everything on the west coast is probably unreachable, so save it for another trip. Just stay in Dublin the whole time and take a day trip by train or guided bus tour to nearby sights on one of those days.

Things to prioritize in Dublin: (1) Trinity College/Book of Kells, (2) Guiness factory (3)) St. Patricks Cathedral and nearby Christ Church cathedral with it's crypt (4) Kilmainhain Gaol prison tour (5) Grab a nightcap in the Temple Bar district (6) St. Stephens Green (7) ride the tram up to Howth in the evening and do the coastal walk then eat dinner at one of the seafood restaurants. (8) if you can squeeze it in, take the bus to Trim to see the castle and cathedral (probably a half day excursion).

Viator has a list of all the guided group tours that operate all day bus tours from Dublin: LINK

Some good ones that cover a lot of ground are (1) Rock of Cashel, Blarney Castle and Cork and (2) Glendalough, Wicklow and Kilkenny.

As mentioned, I would avoid anything that goes all the way to the west coast (Galway, Killarney, Cliffs of Moher, etc.). It's just too far. and I'm guessing 3 days will whet your appetite to make a proper return trip when you can soak up the best of the country.

If you want to plan a day trip on your own, here are some suggestions: (1) train to Belfast where you can take a Black Taxi tour of the "Troubles" sights and have them drop you off at the Titanic museum when finished, then explore city center before taking train back to Dublin. (2) Train to Kilkenny where you can explore most of the town on foot. (3) Train to Athlone and take a Shannon River cruise to Clonmacnois then have a drink at the oldest pub in the world.