Slieve Croob is the tallest of a group of peaks in the middle of County Down, Northern Ireland. These peaks lie north of the Mourne Mountains, between the village of Dromara and the town of Castlewellan. Slieve Croob has been designated an Area of Outstanding Natural Beauty. It is the source of the River Lagan, which starts as a spring and runs from here, through Dromara, Dromore, County Down, Lisburn and Belfast, where it enters Belfast Lough.