George Charles Mostyn, 6th Lord Vaux, bought Derryloughan East, West and South, parish of Burrishoole, barony of Burrishoole, county Mayo, from the O'Donels of Newport in 1854. In 1876 the Honourable Montagu Henry Mostyn [2nd son of George Charles Mostyn] of Rosmead, Delvin, county Westmeath, held 1,460 acres in county Mayo valued at £159. The Mayo estate of the Mostyns, with more than 3,000 acres in counties Westmeath and Meath, was advertised for sale in the Landed Estates' Court in July 1879. The county Mayo part of the estate was obviously not sold as by March 1916 Lord Vaux had accepted a final offer from the Congested Districts' Board for the purchase of 1455 acres.
In 1857 Elizabeth Collum and Robert Orme offered for sale in the Encumbered Estates Court, lands at Kilmain and houses in the town of Athlone, formerly the property of Ffolliott Thornton Mostyn, for whom Elizabeth Collum was acting as executrix.
National Archives of Ireland:
Encumbered Estates' Court Rentals (O'Brien), O’Donnell, 6 July 1854, Vol 30, MRGS 39/013, (microfilm copy in NUIG)