11 days in Ireland...Itinerary help!

First thing - that is peak tourist time, so car hire will be quite expensive - book as early as you can.

As for itinerary suggestions, if you search through here, you'll find loads of suggestions. You can base yourself in Dublin for at least half that time - there is plenty to see in Dublin, and you can base yourself in the city for sightseeing - exploring the Wicklow Mountains, daytrips to places like Kilkenny, etc. As most public transport revolves around Dublin as a hub, it can be easier to explore from the city than hopping from town to town if you don't have a car. Better to save your driving time for exploring the west coast (just bear in mind that car hire companies charge a lot more if you drop off somewhere different from your pick up point).

Anywhere from Cork up to Sligo and Donegal is great for exploring. I'd particularly recommend spending some time around Galway, and spending at least a night on one of the Aran Islands, they can be quite magical.

11 days is a good amount. I would suggest Dublin and surrounds (3-4 days), Galway and surrounds (2-3 days), Killarney (2 days inc. Ring of Kerry), Dingle Penninsula (overnight in Dingle), Cork and surrounds (2 days), back to Dub. If you buy a guide book you can see what they have to offer and if somewhere like Kenmare is more to your liking. It is heavy on the big towns so perhaps substitute Schull or Blatimore for Cork or such.

In terms of historic castles and such you pretty much can't swing a cat in Ireland without thumping it into a ruin or structure a few hundred years old but I would suggest avoiding somewhere like tourist trap Blarney Castle and perhaps looking at things like Newgrange and the Hill of Tara (which a tourist 'experience' are more intelligent). Ashford Castle is gorgeous (although a hotel) and there is a great boat trip you can take form there to monastic islands and Killarney National Park also has some great ruins.

Thank you both for your suggestions! I feel like I have a starting point for planning my trip now! : )

www.budget.ie are doing good deals on car rentals. i got one in june for 7 days 77 euro!
(must be 7 days or more to qualify)

If you decide to travel from Galway to Kerry you should try visiting bunratty castle and folk park just outside limerick city, king johns castle is in the city. Both lovely. Also I like blarney castle, it's a ruin now but you can climb to the top, kiss the stone and walk in the beautiful gardens and visit hhe old house in July. Good Luck!