Glenullin Bog is at the bottom of a big, gently sloping bowl shape in the countryside 4 kilometers south west of the town of Garvagh in Northern Ireland. It is an area called Brockagh, south of the B64 road with the Glenullin Water on its north side and the Agivey river (Glen Ullin) to its south side. These two rivers meet at the eastern side of the bog.

It is about 124 hectares in area and can be seen in the surrounding countryside as a big, brownish purple carpet settled quietly at the bottom of this shallow glen.

The best view of it is from the surrounding higher land,such as from the Glen Road, one kilometer west of the church in Brockaghboy.