Along the north bank of the Fane River in Drumgristin, County Monaghan, there was once an earthwork substantial enough to catch the attention of Ordnance Survey cartographers twice over, in 1834 and again in 1907, each time labelled as the site of an "Ancient Entrenchment". The first map shows it as a linear feature running roughly east to west for about eighty metres, sitting at the foot of a south-facing slope and drawing close to the river at its western end. By the time of the second survey, the feature was recorded more precisely as a bank with a fosse on its northern side. A fosse is simply a ditch, typically dug to accompany a defensive bank, the spoil from the digging often used to build up the bank itself.

A site visit recorded in 1968 found the earthwork still standing, if overgrown. The bank ran about 84.5 metres in length and was noticeably more substantial at the eastern end, where it stood 4.5 metres high on its southern face and carried a fosse nearly eleven metres wide at the top and four metres deep. Towards the western end it tapered away almost to nothing, though faint traces of the bank continued for roughly another twenty-seven metres beyond the main body. By 1995, the structure had been removed entirely. What it once was remains an open question. Writing in 2017, Ó Drisceoil suggested it may have functioned as a promontory fort, a type of enclosure that uses natural features, in this case the river, to defend one or more sides of a site, with an artificial bank and ditch completing the circuit on the more exposed approach. The alignment of the earthwork with the Fane, and the way it nearly met the water at its western end, lends that interpretation some weight, though without excavation it remains speculative.

There is nothing left to see at Drumgristin today. The earthwork that survived long enough to be mapped twice and measured in detail is gone, absorbed back into the landscape sometime before the mid-1990s. What remains is the shape of the argument: a river bend, a slope, and the cartographic memory of something that once stood between them.