This apartment (flat) is within walking distance of central Belfast, St George's market, and quite a few great restaurants. It's perfect for two people, although you could sleep more than that. That said, you Americans (myself included!) will find that places are smaller than what you may be used to here. It's really not a problem, just make good use of your space and you're fine. It has an all in one washer-dryer which is perfect if you are traveling for an extended period. A full kitchen, which means you can buy food at the market and make it there. That will save you some money. St George's market is open Friday, Saturday, & Sunday.
Overall I totally recommend!
One last tip: book certain things online before you go (Giant's Causeway, Carrick-a-rede rope bridge, Titanic Belfast) Belfast is no longer a secret.