This surname is derived from the Gaelic name "Lag(g)an". Some associated spellings are MacGlagan and MacLagger.

The parish of Logierait, generally in Gaelic was called ‘Lagan’ and meant ‘hollow’. The name is derived from the ancient celtic earls of Atholl having had (close to Logierait) their castle (or rath) and where the earls’ court was held for this district. The lands of Glenerochy were known in Gaelic as "Murelaggan" meaning ‘large hollow’. Glenerochy became a possession of the clan through Andrew de Atholia’s marriage to the daughter of Ewen de Glenerochy, son of Conan, natural son of Henry, third and last celtic earl of Atholl. Andrew de Atholia’s father was Duncan, a younger brother of Earl Henry. Andrew’s son, Duncan, is the first recognized chief of our clan.

The name MacLaggan means the sons or servants of Saint Adamnan. This saint of the early celtic Christian church established his church at Dull just north of Aberfeldy. Those lands (area around Fortingall between Loch Rannoch and the north side of Loch Tay) were part of the clan territory. Duncan (1st chief) is reputed to be buried at Dull.

In a charter under the Great Seal dated 1545, Robert Robertson of Strowan was served heir to his father William (6th chief) and at that time the lands of Murelaggan extended from the head of Glenerochy westward to the village of Kinloch Rannoch without interruption.

William (8th chief) had forfeited his estates prior to 1588 because he had alienated part of his baronial estate without the King’s permission on his marriage to a daughter of the Menzies family. On his death, his brother Donald became the 9th chief in 1588 and had a son Robert. Robert (10th chief) was very fortunate. John Robertson, merchant burgess of Edinburgh was able to purchase the forfeited estates in 1599 and had the lands reconveyed to his chief under a new charter under the Great Seal dated January 14, 1600. Under this new charter, the lands of Murelaggan and several others were included within the Strowan barony.

In the rent rolls of 1649 for the parish of Fortingall, the Laird of Strowan Robertson is listed as the landed proprietor of Murelaggan and other estates in that parish.

Some of the crofters and tenants living on these lands may have chosen to be known by the estate name while others might have adopted the Robertson surname. Certainly after the 1745 Rising, many Robertsons did find it convenient (and safer) to adopt a different surname.

James MacLagan wrote the Gaelic words to the music of ‘The Garb of Old Gaul" composed by General Sir John Reid of Straloch. He was ordained a minister in 1760 and became army chaplain of the 42nd Foot (Black Watch) and became the minister of Blair Atholl in 1781. In 1791 it was noted that the pulpit in the Strowan kirk was badly lit and it was recommended to Minister MacLagan to have a sky light of 4 panes of glass installed. This was completed in 1793 for a cost of 15 shillings.

References:
Cameron, Nancy Foy, "Robertsons in Atholl", 1993, p. 88
Kerr, John, "Church and Social History of Atholl", Perth, 1998, p. 68
Kerr, Patricia, "Clan Donnachaidh Annual", 1993, p. 27
Reid, Major J. Robertson, "Short History of the Clan Robertson, Stirling, 1933, p. 95
Robertson, David, "Brief Account of the Clan Donnachaidh", Glasgow, 1894, p. 21
Robertson, James A., "Gaelic Topography of Scotland", Edinburgh, 1869, pp. 445-448
Robertson, James A., "The Earldom of Atholl", privately printed, 1860, pp. 18, 35
Smith, Philip D., "Tartan for Me"