First visit to Ireland

It looks like you plan to rent a car in Dublin and drop it off in Belfast. If that is the case you may want to find out what the one way drop fee will be. It might be considerable. You would be renting the car in one country and returning it to another.

As made perfectly clear by the previous poster your itinerary includes 2 separate countries, the Republic of Ireland and N Ireland. One should read about the Troubles to understand N. Ireland.

Ireland currency is Euros, N Ireland is pounds sterling.

You will need to research costs for the car rental and insurance to cover both countries.

Secondly I’d visit the Dingle Peninsula after Powerscourt and before the Cliffs of Moher. To do this properly I’d overnight in Dingle.

BBC was reporting as of March 4 that Flybe is "set to collapse within hours" and will be shutting down for good. They've had financial difficulties and now with the reduction in passengers with the coronavirus they are flying last flights as of March 4.

https://www.bbc.com/news/business-51746564

Forget Powerscourt and see Glendoulough instead. But, it sounds like a heavy load day 3. I recommend stay in Doolin, Co Clare (great pubs, music) minutes from Cliffs, then lose Roscommon and stay in Galway then to Portrush ( 1 day trip) beach, golf, Giant's Causeway, Bushmills Distillery maybe) DunLuce Castle, then off to Belfast.

In Belfast, be sure to visit the Titanic Museum using the White Star Pass. You get to see the Museum and the SS Nomadic and the discovery tour. Go here to get a feel for it and to book tickets. Time is money, so standing in line for 1 hr. to buy a ticket is time wasted. Buy tickets ahead. https://www.titanicbelfast.com/

Doolin's worth a night and very close to the Cliffs. The Aran Islands are reachable from the port at Doolin and are outstanding.