Rosslare to Kenmare is part of the EuroVelo 1 route if you want to take the scenic route!
https://en.eurovelo.com/ev1/irelandThe first half of the EV1 route to Cork has some detours but are worth while as they are scenic and do not add too much to the route, but from Cork you are really going off track if you stay on EV1, also it does get hilly on EV1 after Cork.
The N25 (National road, equivalent to A roads) is a single carriageway with a hard shoulder so spacious but generally high speed. It is busiest between Waterford and Cork. Thankfully there is the Déise Greenway which is a cycle route from Waterford to Dungarvan on an abandoned railway - totally offroad with a good quality surface and close to amenities. It does get very busy at peak times with families and walkers.
The N25 is also a bit "lumpy" and in parts adds a climbing lane which leaves not much space for cycling.