[1] In the Statistical Accounts of Scotland of 1792, he authored the section covering the parish of Blair Atholl and Strowan in which he voiced warnings about the perceptible weakening of Gaelic in that area.
The Oxford Dictionary of National Biography describes the manuscript collection amassed by McLagan as his "great achievement".
[4] They are largely anonymous and ascribed verse, with a small number of prose items, sourced from many areas of Highland Scotland and also from Ireland and the Isle of Man.
[4] In many instances these manuscripts contain the earliest, or the only, examples of particular poems or songs, and, in the view of Professor of Celtic Derick S. Thomson, they provide "a highly valuable source".
He also used his contacts, both ministerial and military, to acquire versions of poems and songs from other parts of Gaelic Scotland, such as Argyll with its islands, Ross-shire, Inverness-shire, Skye and the outer isles, and districts in Aberdeenshire and elsewhere that were still Gaelic-speaking in his time.
His contemporaries in poetry and song collection included Jerome Stone, a teacher in Dunkeld, Donald MacNicol, Church of Scotland minister of Lismore, Joseph Macintyre from Glenorchy, Archibald MacArthur from Glenlyon, and John Stuart, Church of Scotland minister of Killin (his brother-in-law).
[1] In October 1760, while McLagan was minister of Amulree, he was contacted by James Macpherson, the famous publisher of Fragments of Ancient Poetry collected in the Highlands of Scotland and subject of the Ossian controversy.