I want to know what the best months to visit Ireland are, and what months I should avoid visiting.
There's a saying in Ireland that it can rain twice in one week - once for three days, and again for four days. That's not necessarily true, but it's called the 'Emerald Isle' and the green has to come from somewhere.
Any time from late April through to late September is probably the best option for a holiday weather-wise, but accommodation is going to be more expensive. Ideally I'd avoid the depths of winter though. There can be lovely cold, clear days, but not generally in a row, and there can be rain and sleet too. Last summer there was a heatwave that lasted several weeks over June, July and August, but that's unusual.
The weather is unpredictable coming in straight from the Atlantic Ocean, and it can be pouring with rain in Galway in the west of the island, 130 miles away in Dublin the sun can be 'splitting the stones', and up in Belfast it can be a bit nippy - all in the same day.
July and August are the best time if you are trying to avoid the rain.
We did research, then decided to go August 16th thru Sept 16, so we were there a month and we had four days of sunshine. The locals said its different every year. Definitely go in the summer time because it's the warmest
I suggest don't even try to calculate with the rain. Just accept that it ill probably rain and that's it. Pack waterproof clothes and always wear layered clothing, even when it looks nice outside. Several quick showers during the day are normal, also it can get windy.
I suggest going in the summertime, because even though the temperatures are not that different during summer and winter and it can rain pretty much at any time, Ireland is in the North which means that days are quite long in the summer and very short in the winter (we are talking sunrise at 9 am, sunset at 4 pm...).
I was there in June and though it was a bit cool, it was fantastic! The summer months do not always equal warmth so pack with that in mind if you decide to go in the summer. I was advised not to go in August because that is Ireland's peak tourism month (according to a guide with CIE). There were a fair number of tourists running about in June but it was not overwhelming. The only time it was a bit aggravating was when I went to see Trinity College Old Library & The Book of Kells Exhibition. Then it became a bit oppressive and in the end, due to an argument between an American lady and a French gentleman, didn't get to see the Book of Kells at all. Have fun on your adventure! Ireland is worth every penny.
Mentioned in this answer:
I went in October- early in the month- and it worked pretty well.It wasn't cold and it was less crowded than it would be in the summer.