The whole arrangement bears the name Finvarra's Castle, a reference to Fionnbarra, the king of the Connacht fairies in Irish folklore, whose legendary underground palace was said to lie beneath this very hill. That mythological association did not stop someone in the early nineteenth century from constructing a deliberate architectural folly here, a purely ornamental structure built to evoke atmosphere rather than serve any practical function. The result is a peculiar layering: ancient cairn beneath, romantic fancy on top, folklore threaded through both.

The folly is recorded as dating to the early nineteenth century, a period when the construction of artificial ruins and whimsical structures on prominent hilltops was fashionable among certain landowners and estate improvers. A flight of steps on the north-east side, five of which survive, once gave access to the top of the central structure, though that core has largely collapsed. The two surrounding circular walls, built in drystone technique without mortar, are still traceable across the south-facing arc from east-south-east through to west-north-west, but have disappeared entirely elsewhere. By February 2015, the outer encircling wall had collapsed further along its south-eastern to south-western section, and the resulting rubble had spread three to four metres down the slope of the cairn. A second folly stands approximately 285 metres to the west-north-west, suggesting that whoever conceived these structures had a broader scheme in mind for the hilltop.

Knockmaa is accessible on foot, and the summit rewards the walk with views across north Galway. The ruined folly requires some care to read once you are standing in it; the collapsed sections blend easily into the general scatter of stone, and it takes a moment to pick out the surviving arcs of walling and the remnant steps. The largest of the prehistoric cairns associated with the hill lies about 130 metres to the east-south-east, so the folly and the older monument sit in close proximity, each lending the other a certain ambiguity about what, exactly, you are looking at.