See our guide on travel documentation needed for your little one.
EC Regulation 444/2009:
States that as of June 26th 2012, children travelling from, to or through all Schengen countries plus Bulgaria, Cyprus and Romania are no longer permitted to travel on a parent’s passport, and are required to hold their own individual travel documentation.
Entries on one of their parent's passports are no longer valid. This is irrespective of the age of the child and the passport won’t be accepted as a valid form of documentation.
Henceforth, children travelling to/from any Aer Lingus destination other than those listed below require their own travel documentation, with a passport photo, when travelling.