This depends on where you're travelling to.
Your Irish Passport must be valid up to your return date.
Your Irish Passport must be valid up to your return date and must be an electronic (biometric) passport. An electronic passport is also distinguishable by the symbol on the front of the passport booklet.
You must also apply online for an Electronic System for Travel Authorisation (ESTA) under the Visa Waiver Programme for entry into the United States. This service is run by the U.S. Customs and Border Protection section of the Department of Homeland Security. You can make an application or check the status of your application at https://esta.cbp.dhs.gov/esta/
Those planning on travelling to the US but who are not in possession of an electronic passport (issued since 16 October 2006) will need to apply for a new passport.
If you have received a notification from U.S. Customs and Border Protection that your ESTA has been cancelled, or travel has not been authorized, you may still be eligible to travel to the United States with a visa issued by a U.S. Embassy or Consulate. If your travel is imminent and urgent in nature, please contact the nearest U.S. Embassy or Consulate.
If you received a notification that your ESTA has been cancelled because it is not an electronic passport, but your passport was issued after October 2006 and has the E-passport symbol, please contact the nearest U.S. Embassy or Consulate for assistance.
For current information on visa requirements to the US, please check the US Embassy in Ireland's website.
Further information is available on the DFAT website.
The passport validity required for countries outside of the European Union and the United States varies greatly so contact your travel agent or the embassy of the country you're travelling to for the most up-to-date requirements.