St. Patrick's Stone, as it was recorded during the 1898 to 1899 resurvey of the OS six-inch map, is a glacial erratic, a boulder transported far from its place of origin by glacial ice during the last Ice Age and deposited when the ice eventually melted. This one is substantial by any measure, standing between 2.7 and 4.4 metres high and reaching a maximum width of 7.1 metres, a genuinely massive object resting among the many other boulders that litter the hillside around it.

What elevates this particular erratic above its neighbours is a dark band of discolouration running diagonally across its southern face. Local tradition holds that this stripe marks the line of the chain St. Patrick used to drag the stone all the way from Maam to its present position on the hill. The story neatly reimagines the geological fact of the erratic, a boulder that clearly does not belong where it ended up, as the work of a very specific and very determined saint. The place name Maam, to the north-west in the Joyce Country district of Connemara, roots the legend in real local geography, giving it an internal logic that maps loosely onto the actual movement of boulders during glaciation, just with a rather different agent responsible.