One of the great money-saving features of most credit cards is the availability of Collision Damage Waiver (CDW) insurance for rental cars through your card. By paying for the rental with a card that offers CDW coverage and waiving the coverage sold by the rental agency, you can save a considerable amount of money.
However, through some odd quirk of international law, Ireland is one of an elite group of countries (along with Israel and Jamaica) that most credit card CDW policies specifically exclude, e.g.: http://usa.visa.com/personal/cards/benefits/bft_dmg_waiver_personal.html#anchor_10
This fact creates great confusion and angst to folks renting cars in Ireland, that is only exacerbated by Irish driving conditions. An example can be found on in this Flyertalk thread.
However, some (but not all) World Mastercard cards do offer CDW coverage in Ireland! This includes the Chase Ink Bold card that I happen to carry. By contacting Chase and Mastercard, I obtained a letter of coverage that enabled me to waive the CDW coverage offered by Avis. This saved me at least $150. Thank you Chase!
If you're interested in the card, sign-up links can be found on the pages of three of my favorite bloggers: Gary Leff, Ben Schlappig, and The Frequent Miler.