Lots of cafes have WiFi ( and hotels), there's usually a little sign- you pay a small fee for so many hours. You log on to the website and pay via credit card- it's something like two euro for twelve hours, it counts down automatically. Once you've paid you get a password.
Don't pay more than a small amount at first, sometimes I've had problems connecting.
If all else fails there are internet cafes everywhere too.
I have used an O2 data card over there too but at the time it was very expensive- not sure if that has changed now with the roaming change. It was very handy though as you could use it anywhere. I know some friends use a 3g data card and find it good too.