Belfast
Belfast Lough runs from Blackhead at its north to Groomsport at its south. It is filled by the river Lagan.
Belfast's Busy Harbour
Belfast has formed where the River Lagan meets Belfast Lough. The city itself is named after the much smaller river Blackstaff, which is now almost entirely submerged among the city's foundations. My knowledge of Gaelic does not stretch to the etymology, but you'll find the basics at The Ireland Story, which also includes maps of Ireland back to the ice age!
The mouth of the lough is marked to the north by the lighthouse at Blackhead, and to the south by the town of Groomsport and the Copeland Islands. Northern Ireland's most populous area, this has seen some major development.