Eoghan,
I bought a Eurohike 4 man dome tent fot about £100 five years ago and it's still going strong. It doesn't pack down particularly small or light but it has kept the wind and rain at bay in some big storms in Achill and Easky so it can't be all that bad.
Obviously, if you go for a cool mountaineering brand you're likely to get lighter, stronger, and better feaures, but you have to consider what you are using it for. If Achill in August (with it's unpredictable weather) is the most extreme use you plan, then I wouldn't break the bank. If you want a 7 seasons tent, or to be uber cool and host cocktail parties half way up Everst then apply for your mortgage now.
And as a P.S - remember, it doesn't matter how good the tent is if it is pitched badly, and vice versa: Lorraine and I once went to Strandhill with a tent that cost £19.99 that her mother bought for us in Quinnsworth, and we stayed dry and toasty in an awful storm that blew down Vango's and Vaude's all around us (go me!).