If I buy an iPhone in America will it work in Ireland?

The only place in the US that sells unlocked iPhones is the Apple store. Make absolutely sure you purchase an UNLOCKED (not SIM-FREE) iPhone at the Apple store.

Be aware, however, that if you have problems with the phone, you'll have to send the phone to someone you know in the US for warranty service. Warranty service is NOT international. You should really think this over before deciding to buy a phone in the US and not in Ireland.

so will i know be able to fix my phone over here if it broke?

Correct .

The iPhone warranty is only valid in the country of purchase

You can buy a non carrier locked iPhone from Apple stoere in Belfast that is within the EU and warranty is transportable within all EU countries