I'm going to New York soon, and the iPhone 5C is just $99 USD. I am wondering if an Irish SIM work or if I would have to get it unlocked?
When you buy an iPhone in America for $99 it's probably a SIM-lock together with an subscription. Here you can find the different iPhone 5S models. Search on the internet which Cellular frequencies are used by your provider, and then compare on the site I gave you which models you can use. Mostly the $99 models are locked for AT&T, Verizon,... providers only. So when buying an iPhone in America, make sure you check to model nr on the back of the package.