So it seems Irish has one word for "foot" and "leg." How do speakers deal with the ambiguity? Does it just come down to context? Is there a way to make it abundantly clear which one you mean?
Yes, context is key for cos. Ceann coise unambiguously means “foot”, and géag choise unambiguously means “leg”.
Indeed there is: ceann láimhe unambiguously means “hand”, and géag láimhe unambiguously means “arm”.
Thanks. Do you know how common it is to use it like that? I've never seen it used before so I would guess it's used rarely, but I fell as though most situations when your talking about a cos or a lámh would be ambiguous and call for it.
It’s a well-aged interjection, at least on the western side of the Atlantic, if not on the eastern side — though its use as an interjection would not be literally translated into Irish. (The New English-Irish Dictionary offers Mhuise! or Mo thóin! as translations — the latter is noted as “very informal”.)