Your laptop power supply may be universally compatible (i.e. 100-240V/50-60Hz) in which case all you need is an Irish to US plug adaptor. If not then you actually need a step UP transformer to convert US 110V to Irish 240V. Such an item will probably be bulky and not worth carrying. Might also be expensive. Maplin sell them as far as I know. You might be better off buying a generic universal laptop power supply of the appropriate output voltage over in the US instead if your existing power supply cannot deal with US voltages.
This site might be of interest to you:
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