You will need to bring your own tent(s). You may prefer to all get a single person tent each as this may be simpler for you all to work out your costs. Also, they are a lot easier to put up and take down than a huge tent for multiple people and cost a lot less to buy.
You'll also need a sleeping bag (avoid the mummy shape ones - they aren't as comfortable as you'd think), maybe a roll mat or blow up bed to lie on if you want, consider what you want as a pillow - blow up or a proper pillow or a jumper over your rucksack. That's the basics.
You haven't answered whether you are Irish or visiting Ireland from somewhere else, so can't advise properly on prices, but a basic tent in England is about £20, a basic sleeping bag about the same price, and I take my own pillow - my one comfort thing!
If there are kitchens at a campsite then contact the campsite owners to ask about plates, cups and cutlery. Personally I'd expect to be taking everything of my own to cook with and eat on. Then you need to consider whether you need or want a cool bag/box for milk, fry up material or booze (yes, I know the OP is 17. I'm realistic).
The bathrooms will be very basic, usually there will be loo roll, but that's it. So you'll need to take a towel and all your toiletries of course.
Camping can be done very expensively or very cheaply - I prefer the latter. But then I'm now 30, been camping since I was a small child with the parents and can put up my ten year old, 2-man Eurohike dome tent in five minutes in the dark with pouring rain and gusting winds. It's all down to practice. Before you go, make sure you've practiced putting up and breaking down your tents so you don't look like twits on arrival at the campsite.
Are you going to be driving around or using public transport? You'll need to consider the volume of luggage you'll be taking and how you're going to carry it if you're moving around. Don't want to have an unwieldy and bloody heavy rucksack, plus a tent, plus a sleeping bag, plus a roll mat, plus cooking equipment, plus food.... even between a few of you!
Hope you enjoy it - I love camping.