Sources

terris de Arroquhy-beg cum molendino 1602 Retours (Stirling) 37 [lands of Arrochybeg with a mill]

Milarrochy:
Miln of Arach[..] 1747-55 Roy [cuts off]
Millarach 1807 Arrowsmith
Millarrachie 1817 Grassom
Millarrrachie 1832 Thomson

A few cothouse in one of which there is a Smithy. In former times there was a Mill here. Now the name is always used in the curious form it is authorized here. OS1/32/6/126 [As Millarrochy]

Within the Lennox there are at least five examples of place-names derived from arach, and presumably relating to the arachars:
Arrochar as mentioned above, located at NN 248,044; Milarrochy, presumably ‘mill of the ploughing, located at NS 408,927; Arrochybeg ‘little ploughing NS 414,926; Arrochymore ‘big ploughing’ NS 416,918 and Arachoil ‘the ploughing of the wood’ which survives today as a house name” R. Easson, Systems of Land Assessments in Scotland before 1400, 1986 [PhD thesis]

Arrochy: Errachy 14th C. Lennox Chrs. no. 39

Alternative Forms