Was at the airport and found to my horror I'd brought my expired passport. Luckily it was Aer Lingus and they accepted my Public Service card. No problem on arrival in the UK or checking in to return.
But I got told off by a grumpy Immigration officer. Apparently the Public Service card is not a valid travel document because it doesn't state Nationality. I offered to show my old passport but he just waved me through , once he'd had his moan. At least it wasn't Ryanair - that would have been a disaster. Aer Lingus will accept practically anything with a mugshot . But beware...I just wish the government would make the ID card official. Save lot of grief.