So this is how it works, as set out by the EU: When you reach retirement age you make an application for pension in the country where you are resident. As part of that application you are required to provide details of all EU countries where you made contributions. The French authorities will then contact each country and obtain details of your contributions. They will then do a series of calculations to see how your contributions can be combined to give you the maximum pension. The Irish authorities will then be informed as to how much, if any, they are required to pay you and the remainder will be paid the French authorities. Note that is possible that the entire pension could end up being paid by one authority, if that results in the highest pension!