Lianachan -
Thanks for the link to The Transactions of the Gaelic Society of Inverness. That is an excellent resource.
Note the paper (vol. 23) about the MacDonalds of Achtriachtan (various spellings).
Transactions It seems to have inspired Iain S. Macdonald to write Glencoe and Beyond - The Sheep-Farming Years, 1780-1830. (John Donald, 2005).
In 1998 a book entitled The MacDonalds of Glencoe was written and published by Alexander MacDonald, a local man. On page 13 Alexander has included a map of the Glencoe lands showing the old estate boundaries. Note that the lake in the glen is called Loch Triachtan. Local Gaelic speakers never refer to the lake as Loch Achtriochtan, a name which has wrongly appeared in modern O.S. maps, including the O.S. map in your Gazetteer of Gaelic Place-names.
Ainmean-Àite na h-Alba - Gaelic Place-Names of Scotland - Mapa
To confirm the point, in a Timothy Pont map (c.1590) it is called Loch Trychardan.
Cairdean is the Gaelic word for friends or relatives. Hence, presumably, the adjacent Three Sisters of Glencoe. Also, 'Try' is pronounced as 'tree' in English. Clearly, the letter y represents, for Timothy Pont, an 'ee' sound confirmed by his use of the letter y in nearby Loch L
yon (Lee'on) in the same map.
http://www.nls.uk:8080/StyleServer/c...e&plugin=false
The lake is still being called Loch Triochatan in an O.S. map surveyed in 1867-72 and reprinted in 1987.