Both phrases are new to me, Anon, but I suppose yes, the "cow cabin" is presumably a stall for cows. The "barley axe" is more troublesome; I would be surprised to discover that even 18th century Devon folk cut barley with an "axe", and were then likely to use it on trees. So perhaps it means "the axe that would have cleared the area in readiness for planting barley".
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