The site at Forthill is a rath, an early medieval earthen ringfort of the kind once scattered across Ireland in their thousands, typically serving as the enclosed farmstead of a single family or small community. What makes this one quietly curious is the degree to which the landscape has simply absorbed it. The surrounding grassland has continued to be worked, the old boundary bank has been pressed into service as a field wall, and the interior has been divided by later fencing running on crossing axes. The monument persists, but only just, folded into the ordinary business of farming.

The enclosure is oval in plan, roughly forty metres on its north-south axis and thirty-five metres east to west. Its earthen bank varies considerably as it traces the perimeter. On the north-east, it has been deliberately built up to compensate for a natural slope in the ground, while on the east and south-east it has degraded to almost nothing. The most substantial surviving section runs from the south-west around to the north-north-west, where the bank reaches an external height of about 1.35 metres and a width of around three metres; this is the stretch that was adapted as a field fence, giving it a secondary life that has simultaneously preserved and altered it. A narrow gap on the east side, where the bank is reduced to a low scarp, may mark the position of the original entrance. To the north, the ground falls away gradually towards a natural hollow, now wet with rushes and cut by a drain, and it is in this direction that the site commands its most extensive views across the undulating Mayo countryside.