Sweden is a large country, and more to the point the distance between the southernmost and northernmost parts is over a thousand miles, so there is quite a difference in weather between the two extremes and it's difficult to say anything about the country as a whole. But if you take Stockholm, which is maybe a quarter of the way up the country, and compare that against Dublin, then the summer temperatures are very similar, with typical daytime temperatures in the summer months around the 20C mark. Dublin is very slightly cooler, but only by about a degree or so. (There is a far bigger difference in the winter months, with Stockholm much colder, but your question was only about the summer.)
Other than that, very difficult to say which is 'better to move to', as that can depend on so many factors. Ireland is of course English-speaking so you're likely to get by easier (I'm assuming you don't speak Swedish?), although English is commonly spoken in Sweden and Swedish isn't a particularly difficult language to learn.
Sweden has hot summers. This past summer it was over 30C for weeks. You don't get long stretches of hot weather like that in Ireland. Even in northern Sweden it can get into the high 20s in the summer for weeks at a time while in Ireland it's a hot day if it gets over 25C and you rarely get more than a week stretch that is 25C or higher
If you are not an EU national, it's highly unlikely you'll be moving to either country.
I live in Ireland, and today 21 December it's about 12 degrees Celsius outside. Ireland doesn't usually get very cold in winter due to being located smack-bang in the middle of the Gulf Stream.
Artemisia is wrong. Sweden has a very hot reliable summer. Ireland is temperate all year round.
My guess is Sweden being located further north.
sweden.
