You said your two favorite months in Ireland are May and September.... these are the exact two months we are trying to decide between. We would be traveling mid-month in May or last week of September into October. You described a May with flowers, baby lambs, etc but what is so unique in Sept-Oct? We come from East TN so we are spoiled with Fall harvest and leaf-turning.... I want the full "green" effect of Ireland. Please advise any further thoughts.
September is often given as its the end of the main tourist season (July/August) which also includes the main school holidays here. The end of September is really the end of many things, B&B's start to close for winter and many tourist sites also close.
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Two rules for Ireland, Take your time & bring a sense of humour
May is my very favorite month in Ireland. Everything is fresh, green, blooming and the B&B owners are fresh for a new season too. Generally in September the leaves aren't changing yet...maybe going into Oct. I have an article on my blog: Autumn in Ireland. Everything is green in both seasons. I recommend May.
Another vote for May here. If you go in early May you can take advantage of some of the May bank holiday weekend and some of the things going on then like the Cuckoo Fleadh music festival in Kinvara. Later in May you get the advantage of being close to maximum daylight hours. I've almost always have gone in May and have never found it too crowded anywhere.
If I'm not too late can I enter a vote for September please? I just returned from my sixth trip to Ireland - 5 of them have been taken in late September / early October. I love travelling at that time of the year and find Ireland to be wonderful then. The weather always seems to cooperate. I know that's not a guarantee but this past trip our first week was sunny every day with temps in the 70's. An added bonus when we were there this time many of the trees had started to turn so the fall colors just added to the scenery. I have never had a problem with finding B&B's or attractions closed. And with the summer crowds gone everything is less crowded so I find everyone has more time to chat and you get to know the locals better.
I have never been to Ireland in May - maybe next time. But I think Ireland in the fall is ideal