According to the information in the NISMR, this is a portal tomb consisting of a cairn 27 x 10 x 2m, containing three cists. The one at the east end is large - 6ft by 4ft - and was accessed through two portal stones and a sill.
... this region is rich in places associated with the Patrician mission. A Neolithic chambered cairn on the south-west summit of Knockroe, to the south-east of Clogher, is called Carnfadrig [Carn Phadraig, 'Patrick's mound'], and due east of this, at Altadaven, is St Patrick's Chair and Well, the latter comprising a large cupmark in a rock.
p129 in 'Ireland: An Oxford Archaeological Guide to Sites from Earliest Times to AD 1600', by Andrew Halpin, Conor Newman (2006).