ireland in late July

This will answer your weather question: http://www.wunderground.com/global/IE.html
You should scroll through the "To the North" section and packing for Ireland question has been brought up many, many times. Most every one says to make sure that you have a good raincoat...

I've been to Ireland twice -- in July and August. It can be hot, pleasant or cool. One trip it was mostly sunny; the other it was mostly rainy. I took long pants, a few t-shirts and a few long sleeve shirts. I also took a fleece for when it was cool. And the one thing you must have is a lightweight waterproof (not water resistant) jacket with a good hood. Rain is always a possibility in Ireland, but it's likely to be a light rain or drizzle that doesn't interfere so much with sighseeing.

We visited a few years back in early August. No way to accurately estimate for sure as the weather in Ireland is extremely changeable.

When we were there, it was a very mixed bag. For 2 days in Dublin it was very hot (upper 80's) and sunny. Hotel didn't have air-conditioning and not much breeze for night sleeping. By the time we hit the Cliffs of Moher, it was cool (light jacket) and dizzily to the point we couldn't even see the Cliffs. Warmed up again when we got to Galway. Generally alot of climate changes even throughout the day.

I know this doesn't help alot for packing, but it's just the way it is (was).

I would recommend one of those compact folding rain ponchos with hood that you can keep with you. We were at the giant's causeway in late May. It was beautiful and sunny when we left the visitors center; sleeting by the time we reached the causeway and had cleared up by the time the shuttle came. Keep an eye on the sky as you can see the clouds roll in with their moisture

The more north, and west you go, the more unpredictable the weather gets.

My late July trip there was alternatively like a Seattle November, or May.

During my week long trip to Ireland it only rained twice-once for four days and once for three.

I believe Niall Tobin deserves the credit for this bit of Irish humor.