Between 1939 and 1941, he taught at Boroughmuir High School in Edinburgh, and in Hawick. During this period, he wrote most of the poetry that would become "Dàin do Eimhir", including the epic "An Cuilthionn". MacLean cultivated friendships with Scottish Renaissance poets, including MacDiarmid, Robert Garioch, Norman MacCaig, Douglas Young, and George Campbell Hay. MacLean, also a noted historian, published two influential papers on nineteenth-century Gaelic poetry in "Transactions of the " in 1938 and 1939, which challenged the Celtic Twilight view of Gaelic literature. MacLean accused the "Celtic Twilightists" with "attributing to Gaelic poetry the very opposite of every quality which it actually has", at which they only succeeded because they catered to a credulous English-speaking audience. He pointed out that the apparent sentimentality or impotence of the corpus of Clearance poetry may have been an artefact of the fact that landlords would not have preserved poetry critical of them. His use of Gaelic poetry as a potential source material for historical studies was extremely innovative at the time.