Try "highly variable and unpredictable". It also varies considerably depending on whether you are in the north or south. Cork is never very cold by Canadian standards, and on any given day you will usually get some sun, some cloud and some rain. I once had my own personal rainstorm as I crossed the quad, just like something out of a cartoon, and when I flew into Dublin last year, as I was waiting for the bus to leave for Cork I looked out my window and saw that it was raining on my side of the bus, but it was still sunny and clear on the other!
In the winter there does tend to be more rain than most of us are used to, but on the plus side, no snow worth speaking of. Bring a rain coat and rain pants - Goretex preferably. Umbrellas are of limited use in Cork since it is usually windy when it rains - not sure about Dublin but suspect it is similar. Buy a cheap collapsible umbrella when you get there, cheap because you will either leave it somewhere or it will get turned inside out and wrecked within a few months, and collapsible so you can put it in the bottom of your pack for when you need it.
People just adjust to the rain. When I first got to Ireland I ran for cover when the rain started, and was amazed to see people just trudging through it, but now I do the same. No point in running for cover everytime it starts to rain - you'd spend most of your time in doorways. And you'll dry off, eventually.
Just had the hottest summer in living memory. The grass actually scorched , damn hot for Ireland . Weather usually cloudy though , sometimes rainy, sometimes sunny however mostly grey . Hardly ever snows, maybe 1-2 days a year in Jan or Feb (so skiing is out except on the carpet mountain yoke). Sunny usually around exam time may june depending. Never really that hot and never really too cold .
Ireland can get pretty damn hot. I stayed last summer and I nearly changed race, I was so tan. And good thing is, it doesn't get as cold as the US, although the kind of cold is different. It's a damp going-through-your-bones kind of cold.
Galway and Cork get a lot more rain than Dublin, being along the West and South Coast respectively, so stay clear of them if you don't like the rain. The weather has been fantastic all over the British Isles this Summer; temperatures even reached 30C in Aberdeen!