Belfast is the capital city and principal port of Northern Ireland, standing on the banks of the River Lagan and connected to the open sea throug…Belfast is the capital city and principal port of Northern Ireland, standing on the banks of the River Lagan and connected to the open sea through Belfast Lough and the North Channel. It is the second-largest city on the island of Ireland, with an estimated population of 348,005 in 2022, and a metropolitan area population of 671,559.