Yes, you are overdoing it.
Doing Dublin to Newgrange to Hill of Tara to Cashel is not impossible, but its a huge journey and involves having to come back to Dublin (from Meath to Tipperary means having to use the M50 orbital around Dublin). Its about 250 km of driving, which is a lot on Irish roads. Also, there can be very long waits to see Newgrange, depending on the time of year you travel. I would suggest instead (assuming you really want to see Cashel), to do a route that touches on the nicer places to see between Dublin and Tipperary. Perhaps go to Glendalough in Wicklow (a very lovely place for walks and history), and stop off in Kilkenny on the way to Cashel. Much less driving, and just as nice, if not nicer (the road over the Wicklow Mountains can be very beautiful.
Cashel to Kllarney via Cobh and Blarney is.... well, very long, It also involves nearly 250 km of driving (check out www.aaireland.ie or googlemaps for route planning). And neither Blarney nor Cobh are worth it - really. Blarney is a horrible tourist trap and Cobh is a pleasant coastal town, but by no means the nicest in Cork, and to get there you have to fight your way through Cork city traffic. If you want to see parts of Cork on the way to Killarney, then I'd suggest driving from Cashel down to Kinsale (much prettier than Cobh), then go through some of the towns of West Cork (Clonakilty is nice, Castletownbere is in a magnificent setting), before going to Killarney. But I'd advise Kenmare above Killarney (less twee), and if you have time, visiting the Dingle Peninsula on the way up to Ennis (don't go through Limerick, take the ferry at Tarbert instead).