The honest answer is that Ireland absorbs as much time as you give it. The useful answer is that there are natural breakpoints, and knowing them stops you building a trip that fights itself.
Five days
One region, plus the city you fly into. Not two regions. Not a loop. Pick the southwest, the west, or the north, and do it properly. Two bases, maximum three.
Five days is a genuine trip. It is not a short version of two weeks, and building it by subtraction from a fortnight is the commonest planning mistake we see.
Seven to eight days
The most common length, and enough for one region done thoroughly plus a couple of days in Dublin. Or, if you skip Dublin, two adjacent regions at a civilised pace — Kerry and Clare, or Galway and Connemara.
Three bases is the ceiling here. Four means you spend a day of your week checking in and out.
Ten days
This is where the island starts to open up. You can do Dublin, the southwest, and the west without rushing. Or the whole of the Wild Atlantic Way at a fast clip. Or the Republic’s greatest hits plus a genuine two nights in Northern Ireland.
Ten days is our recommendation for a first trip if you can manage it. It is long enough to have a slow day without feeling you have wasted one.
Fourteen days
The whole island, thinly, or two regions with real depth. Both are valid; they are different holidays.
A fortnight is also the point at which a rest day stops being a luxury. Everyone who drives Ireland for two weeks without a day off comes home tired.
Three weeks or more
Now you can include the places that get cut: Donegal, the Beara Peninsula, Achill, the Aran Islands overnight rather than as a day trip. You can also afford weather. If a day is filthy, you do something else and go back.
The number that actually matters
Not days. Bases.
Divide your nights by the number of hotels and the answer should be two or more. If it is 1.2, you have planned a moving-house exercise, not a holiday. The single change that improves most Ireland itineraries is deleting a destination and giving those nights to the ones either side.
A rough rule
Allow two nights per region minimum, one full day per major drive, and one spare day per week for weather and for the thing you did not know you would want to do. Then see how many regions that leaves you. It is usually fewer than the plan you arrived with, and the trip is better for it.