The majority of Belfast is located in County Antrim, but some of South and East Belfast are located on County Down. Belfast is the 15th largest city in the United Kingdom and the second-largest on the island of Ireland with an estimated 2014 population of 295,000.hile the 2014 population of the city proper is 295,000, the Belfast Metropolitan Area is home to 585,000 people. This makes it the 15th largest city in the U.K. and the 11th largest conurbation.