Irish Walking routes and other great places to see.
Except for very unusual weather cold is not much of a problem here in Ireland. In the winter check the local weather forecasts. Wind is something that many visitors to Ireland notice a lot more than any other weather condition. It is windy, so bear in mind the wind chill factor all the time especially towards the end of the day when one is getting tired or on the top of un-protected hills and flat land.
What you pack very much depends on where you decide to stay. Camping will necessitate the most amount to be carried, hostels less, Bed and Breakfasts, and Hotels the least amount.
You should always have good waterproofs, it is nor that it rains a lot, but can rain for ten minutes and then be sunny for the rest of the day, but rain showers are to be expected - sometimes fairly heavy. I hate to carry any wet clothes and therefore I always use a liner inside my rucksack and a waterproof cover on the out side. Carry lots of water and some food.
You will have to decide on what extras to take, but keep it as light as possible. Though during the early spring and late autumn it can get very cold with strong winds adding to the chill factor. Different guide books suggest different lists, however the most important is keeping dry, warm, safe and as light as possible.