It’s a simple question. The answer is “probably”!
To explain a bit more: I went to Dublin today for work. I flew Ryanair from Stansted, and presented my UK driver’s licence when asked for my ID at the gate. “Sorry, we don’t accept those”, said the gate agent. Fortunately, I had also brought my passport, knowing that whilst it’s theoretically possible to go to the Republic without a passport, Ryanair are nearly always arsey about these things.
The UK and the Republic of Ireland have something called the “Common Travel Area”, which is like a watered-down version of the Schengen agreement that allows passport-free travel in most of mainland Europe. Unlike Schengen, it came about not from a high-minded view of the free movement of people, but more from administrative convenience (how British!). When southern Ireland gained independence in 1923, the UK government was extremely reluctant to introduce and police a “hard” land border across rural Ireland, and so ever since we’ve had arrangements that you can cross the land border without a passport. During the Troubles, of course, there were checkpoints, but since the peace agreement the land border is just a roadsign. If you travel between the Republic and the UK by sea or air, you are not technically required to have a passport – just some proof of your ID. Unfortunately, the specifics of this are left to the carriers, and so they all implement slightly different policies. Ryanair, predictably, have taken the option that is most convenient for them, which is simply to insist upon seeing everyone’s passport. So, your cheap flight with Ryanair may “have cost you 50p” but you’ll need to shell out £72.50 for a passport if you don’t have one already! All the other airlines will accept some other form of ID, and a draft bill was presented to the Irish parliament to require that carriers accept alternative ID – but it failed to pass.
On arrival at Dublin airport, arrivals from the UK are treated the same as all other international arrivals: you queue up for immigration and present your documentation to the officer on duty (or use the electronic gates if your passport is compatible with them). So, I can enter Ireland by land from Belfast and not show documentation, but if I arrive by air from the UK then I must. Even more unusually, the situation is not symmetrical – upon arriving at Stansted, all passengers from the Republic are bussed around to a special arrivals door on the end of the terminal (also used for domestic arrivals), which leads directly to the baggage reclaim and avoids the immigration control.
Next time you go to Dublin, take your passport.