It does depend on how much time you have.
Going from Rosslaire around clockwise, through Cork, Kerry, Limerick, up through Galway, Sligo, Donegal is definitely a stunning trip, but would take a few days.
The roads are mainly good, and the scenery is stunning. But it is a trip in itself and needs to include some detours off the main city to city roads to be appreciated.
Failing that, going Rosslaire, Waterford, Portlaoise, Athlone, up along the Shannon to Carrig on Shannon, then Enniskillen, Derry can be a good trip. Again as Armstrongracer said, you are sometimes not going on main roads. But the B roads are not that bad. If I was doing it, I'd break it into two days.
We can give you more information on sights, and recommended stopping points when you decide a route.