The Lagan rises in the Dromara mountains in Co Down on the northern face of Slieve Croob (Mountain of the Hoof) it wends its way generally northwards towards the village of Donacloney where its waters were once utilized to power the linen industry from there its turns to the east past the industrial town of Lisburn in Co Antrim and thence to Belfast where it meets the sea

The river was once part of the Ulster Canal carrying cargo from the southeastern corner Lough Neagh to Belfast, an act of vandalism perpetrated by the planning authorities in the mid 1960's allowed the canal to be filled in to build the first section of the M1 motorway from Belfast to the west of the Provence.