Although the late mediaeval "Hystoria Fundacionis Prioratus Insule de Traile" claims that Fergus founded the priory of St Mary's Isle, the fantastical foundation claims preserved by this source are not corroborated by contemporary sources. According to a confirmation charter dating to within the decade after Fergus's death, Fergus granted the house to the abbey of Holyrood. A confirmation charter of William I, King of Scotland reveals that the priory of St Mary's Isle may have been in existence by the time of Fergus's grandson, Roland fitz Uhtred, Lord of Galloway, although the first recorded prior appears in the thirteenth century. Fergus's supposed links with this house, therefore, are dubious. Although Walter Bower declared that Fergus was responsible for the foundation of the abbey of Tongland, his great-grandson, Alan fitz Roland, Lord of Galloway, appears to have founded it in the thirteenth century. The erroneous attribution of Fergus to this house may be the result of an attempt to enhance the antiquity of its establishment by linking it with the progenitor of Alan's family.