As far as I am aware, both Ireland and the UK will allow you to work. France, Germany etc will not. Not sure how it works with Irish registered aircraft, but since the initial period of work with Ryanair is classed as training, this will mean you aren't getting paid.
Wizzair in Poland has French & German nationals working under their TR training scheme, and the Poles don't allow the French or Germans the right to work in Poland, so it must be classified as 'training'.