Hi guys, from 'Mingulay, an island and its people' by Ben Buxton, p.124:
"Vatersay was run as a single farm, occupied by the tenant farmer and his workers; it had been crofted before 1850, when the people were evicted, and had been designated as suitable for crofting by a government commission in 1894. ... Some of the cottars from Barra were descended from the former inhabitants, and had continued to bury their dead in the graveyard at the south end of Vatersay. ... In 1883, forty-five Barra cottars, living in wretched conditions and eking out a meagre existence by fishing, applied for holdings on Vatersay, but were turned down by the landowner, Lady Gordon Cathcart. ... Further appeals failed, including one from Barra Parish Council, and the men grew desperate. Then, in September 1900, emboldened by the success of cottars raiding the farms on Northbay and Eoligarry in Barra, the men raided Vatersay, though they subsequently withdrew.'
I'm guessing that Archibald and Mary did indeed work for Lady Cathcart's farm. Having been born of families from South Uist, they would indeed have been thought of as 'strangers'.