Make sure to wear Hiking boots, everything else is lethal.
Then bring a stick, it'll help you on the way down. (You can rent or buy them at the car park as well! It costs IEP 1.50 to rent and IEP 2 to buy or something...)
Don't forget to bring lots of drinks (non alcoholic please!!!!) And leave your cigarettes in the car ;-) Then you have something to reeeaaally look forward to on the way down ;)
Read my Croagh Patrick Travelogue for more important tips!