So it's sensible to carry your passport. Doing so may help you to avoid difficulties and get past bureaucracy. When I'm overseas I carry my passport, or a photocopy of the crucial page from it at least, everywhere I go, though technically I don't have to. The only other photo ID in the whole world that I possess is my bus pass, which no-one recognises as valid ID. I still have my tatty old paper driving licence, sans photo, issued to me last in 1976 if I remember correctly (I really must look for it some time). It bears a huge tea stain and a handwritten record of my 1990 speeding rap (bastard Wiltshire cops). But passports for UK and Irish citizens travelling between the two countries are never a legal requirement. If you don't want a fight at the Ryanair departure gate it makes a lot of sense to have it handy. I prefer to sail through such obstacles rather than make principled stands. There you are, the alpha and omega of it.