Don't Easyjet already have a Swiss subsidiary for similar reasons?
As for Ryanair, various agreements on trade, business etc between Ireland and the UK pre date the EEC or EU. One has never needed a passport to travel between the 2 countries, nor has there evere been full border checkpoints between The Republic and Northern Ireland. This may change if the UK leaves the EU however, as one of the main reasons to leave is more control of immigration. If the UK leaves the EU then EU citizens can travel freely to Ireland and hop across the border. Introducing a full border would give Republican extremists an excuse to start bombing again sadly.