A lot of apartment blocks have gone up in Belfast in the last five to ten years. The rental market is not as buoyant as the market in Dublin. Salaries tend to be lower and there are still very cheap rental properties to be had in Belfast. Very often the rents in these apartment blocks are quite high relative to rental properties else where in the city. Traffic in Belfast is entirely different to Dublin and so a lot of people who work in the city centre travel into work from other areas of Belfast and surrounding towns and so don't need to live in town to keep their commute down. Rates in Belfast at the moment are very high - £1000 a year on a two bed apartment worth £180,000 for example. That's all the negatives I can think of.
On the positive side Belfast has changed enormously in the last ten years and property prices are on the way up. Interest rates though are a good bit higher than euro rates at the moment. I wouldn't be against investing in Belfast and I've done it myself, but it's the kind of town where you really need to know where you're buying.