The River Lagan is a major river in Northern Ireland which runs 53.5 mi (86.1 km) from the Slieve Croob mountain in County Down to Belfast where it enters Belfast Lough, an inlet of the Irish Sea. The Lagan forms much of the border between County Antrim and County Down in the east of Ulster. It rises as a tiny, fast-moving stream near to the summit of Slieve Croob; Transmitter Road runs nearby. It runs to Belfast through Dromara, Donaghcloney and Dromore. On the lower slopes of the mountain, it combines with a branch from Legananny Mountain, just opposite Slieve Croob. The river then turns east to Magheralin into a broad plain between the plateaus of Antrim and Down.

Slieve Croob is a mountain with a height of 534 metres (1,752 ft) in the middle of County Down, Northern Ireland. It is the heart of a mountainous area, the Dromara Hills, north of the Mourne Mountains. It is designated an Area of Outstanding Natural Beauty and is the source of the River Lagan. There is a small road to the summit, where there is an ancient burial cairn and several transmitter stations with radio masts. It has wide views over all of County Down and further afield. The Dromara Hills also includes Slievenisky, Cratlieve, Slievegarran and Slievenaboley.