I have an american iPhone and even though bought 20mb plan for international that goes fast considering the data hogging nature. Good thing Dublin has plenty of free wi-fi spots. Even the gravity bar at the guiness brewery gives you ten free minutes which is plenty of time to check email and update the new york times. The Supermac on O'donnel street is a convient place to get free wi-fi. Most hotels including the budget one I stayed at provide free wi-fi. Only time used internet cafes was to steal some power from the USB since iphones come with handy USB charger ability.
Fondest memory: Ease of walking around and friendliness of people though having map feature and gps on iphone comes in handy since streets can change names like every three blocks in this city.