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Plas Machynlleth was the Welsh residence of the Marquesses of Londonderry. It is situated in the market town of Machynlleth in Powys (formerly Montgomeryshire), Wales. It was brought into the family by the marriage of George Henry Robert Charles William Vane-Tempest, Viscount Seaham (and subsequently Earl Vane and 5th Marquess of Londonderry), to Mary Cornelia Edwards, whose father Sir John Edwards had extended and renamed the house. It is a Grade II* listed building. 
The house was the family home of the 5th Marquess. His eldest son left Machynlleth on succeeding to the Marquessate, but his youngest son, Lord Herbert Vane-Tempest, remained resident at the Plas until he was killed in the Abermule train collision on 26 January 1921.
The oldest parts of the house date back to the seventeenth century; the main entrance front was added in 1853. The house was called Greenfields for many years. It was later renamed after the town.
The 7th Marquess of Londonderry gave the mansion and its estate to the town, and the house was used for council offices. In 1995, after a £3 million refurbishment, funded by Montgomeryshire District Council and the European Union, the building became the "Celtica" heritage centre. For several years the centre was successful in attracting tourist and educational visits and conferences. It was taken over by the new unitary authority, Powys County Council. With little investment by the Council and declining visitor numbers the Council decided to close the centre in 2006, citing a loss of £1.1 million between 1998 and its closure. The Plas is now used as a community and meetings venue.
Other residences of the Marquesses of Londonderry:
- Londonderry House in London
- Mount Stewart in Northern Ireland
- Seaham Hall in County Durham
- Wynyard Park in County Durham
- Loring Hall in Kent
- "Plas MacHynlleth,plas Drive, Machynlleth". British Listed Buildings. Retrieved 7 August 2015.
- BBC News (6 September 2005). "Celtica Attraction to Close". Retrieved 2007-06-13.