Ireland is infamous for its rain. The bad news is that, yes, it does rain quite a bit. The good news is that we do have some sunshine, even if we always look a little pale. And the rain and mist are an intrinsic part of Irish mysticism.
Umbrellas may not be part of this romantic ideal, but they are very much part of the practical, as are jumpers, jackets and the odd pair of woolly socks!
In winter, temperatures fluctuate between freezing, cold and bearable - anything from -5°C to 15°C. In summer, temperatures are bearable to hot- in the region of 15°C to 25°C. You'll need two different wardrobes, but you'll always be mixing and matching.