This motte and bailey castle, likely constructed by the Anglo-Norman knight Hugh Tyrel between 1186 and 1223, once commanded impressive views across the surrounding countryside. The motte itself, though now heavily eroded and partially destroyed by the road that cuts through its southern side, was documented in 1910 as a classic pudding bowl-shaped mound standing about 5 metres high with a summit measuring 20 by 12 metres. Today, vegetation cover and soil slippage have considerably altered its profile, with visible scarring along the sides where the earthen core has been exposed.

The summit bears traces of what appears to be a former Ordnance Survey trigonometrical station, whilst 20 metres to the east stand the ruins of a later stone castle, built upon what may have been the original bailey or courtyard of the earth and timber fortification. This raised area, measuring approximately 55 by 41 metres, was likely repurposed as a courtyard when the stone castle was erected. A significant depression in the northeast quadrant might mark the location of a mansion house recorded in 1826, possibly a gatehouse that controlled access to the bailey’s interior from the north.

Hugh Tyrel’s significance in the area is well documented in medieval records; in 1223, King Henry III formally recognised his landholdings in Ireland, and by 1232, Tyrel had been granted the right to hold a yearly fair at his manor of Newtown in Fertullagh parish. This manor was probably centred around modern-day Tyrrellspass, about 5 kilometres south-southwest of Castlelost. The proximity of a medieval church and graveyard 400 metres to the north suggests this was once a thriving Anglo-Norman settlement, strategically positioned to control the surrounding territory during the early phases of English colonisation in Ireland.