We are planning to fly into Shannon rent a car and spend the last 3 days in Dublin before we fly out from that airport. Should we return the car when we arrive in Dublin and use public transportation to get around Dublin ? Is driving and parking a headache, much like in NY or Philly ? how expensive is parking?

Unless you are staying outside Dublin, you will not need nor want a car. So if you are staying in Dublin, return the car and be car-free and care-free for the last 3 days.

Driving and parking are definitely issues in Dublin. A car is not necessary and, actually, a liability.

Only mad dogs and Irishmen try to drive around Dublin. Ditch the car ASAP.

Most of the main sites in Dublin are walkable. We used a hop-on, hop-off bus to get around to some of the more distant sites. We had a rental car that we just parked for the 2 days we had in Dublin.

Get rid of the car as it would be similar hassle to NY/Philly. Some things are the same in every country