The Guinness Storehouse is on the top of every "Dublin" travel list and I agree. However, pay extra for the Connoisseur Experience. You will be taken to a private tasting area that resembles an early 1900's study and be treated to a unique tasting experience of Guinness not only as you know it but how it is served in other parts of the world.
Rent a car and drive the Ring of Kerry. Stops along the way should include Muckross Abbey and Ross Castle. We stayed at Gleann Fia Country House in Killarney and were treated to traditional Irish meals and wonderful hospitality.
Don't travel to Dublin over Christmas. While we had an amazing trip, most of major sights were closed for the holiday until the New Year.