Looks good! A few flight related items to consider:

If you haven't already booked your flights yet, be sure to check both LCCs and mainline carriers for the Paris-Dublin leg. Ryanair uses Beauvais, which is a solid 50miles from downtown Paris, and the transfer to the airport is a pain. I've found that Aer Lingus often isn't that expensive, and has a much more convenient CDG departure.

On the BCN to NCE leg, I don't think there are any LCCs, so that might be an expensive segment if you want to fly. It's an easy train ride, though.

In terms of where to spend your extra day, personally I might suggest somewhere in Ireland, and use that time to get out of Dublin and check out a small town. I as bcmatt noted there aren't tons of tourist attractions in Dublin, and a lot of the charm of Ireland is in the small towns.