Glenullin Bog is an ombrotrophic mire (a mire nourished by rain water only). Lowland ombrotrophic peatlands have been described in Britain and Europe by Osvald (1952), and Moore and Bellamy (1973). A continuum of different types exists across Europe as a result of climatic and geographical variation.

Glenullin Bog is a raised bog with a distinct cupola (dome). This equates closely with the ridge raised lowland bogs described in Ireland by Bellamy (1986).

These peatlands developed under the influence of the Irish oceanic climate. They are structurally and botanically unique both within Europe and worldwide. In an undisturbed state they have a cupola and a surrounding lagg area with typical fen type vegetation.

Past exploitation of peatlands for fuel and industrial materials has caused a 94% loss if Irish raised bog habitat (IPCC 1992). The area of intact raised bog remaining in Northern Ireland is estimated at 2279ha. This is only 9% of the estimated area which existed before exploited (Cruickshank and Tomlinson, 1986). Raised bogs retaining any remnant of lagg vegetation are now extremely rare in Northern Ireland. Only 12 examples were recorded by Leach and Corbett (1987)

The Glenullin Bog cupola has been reduced by hand cutting at its margins by approximately one quarter. The area remaining is roughly 40ha. This is a large area by Northern Irish standards. Only 4 bogs with an intact area greater than 100ha were recorded by Leach and Corbett (1987). Only six bogs of sixty-three visited in this survey were found to have more than 75% of the total area intact. The cupola of Glenullin Bog is equivalent to 1.75% of the intact lowland raised bog resource in Northern Ireland remaining in 1986.

Glenullin Bog lies to the north and west of the Glenullin Road. It consists of a semi-intact cupola of uncut peat and surrounded by turbary cuttings.
These turbary cuttings are extensive on the south west and north east sides. The bog edge adjacent to the Glenullin water in the north and the Glenullin Road to the south retain a more natural margin (Comm. D. Hughes, 7.6.94). The cupola is described as semi-intact due to the recent perturbation of its surface by an illegal commercial operator (McDon Peat) who carried out some drainage and vegetational removal. The primary morphology of the bog remains in place. The cupola is still present and has retained its domed structure despite the attempt by McDon Peat to destroy it. Initial measurements suggest that it is 3m higher in the centre than its southern edge at the Glenullin Road.

The bog is bound by the Glenullin Water on its northern and eastern edge. The western boundary is less distinct as this is where much of the past hand cutting has taken place. A major drain and raised bank (probably a former access bank for peat cutters) transects the bog between the Glenullin Water and Glenullin Road along the western side. The dome of uncut peat lies to the east of this bank. To the west lies a cut-over area which extends towards the Lisnascreghog Road. Some of these cuttings have been reclaimed for agriculture, but the majority have regenerated with ombrothrophic (raised bog) or fen vegetation. In the other areas of the bog the former cuttings have been left undisturbed since cutting took place except in a few areas where they have been reworked by a Difco type (sausage) machine cutter.