"She is not wanting a stone."
Translation:Chan eil i ag iarraidh clach.
That's a preposition, so the progressive aspect is technically "to be AT doing" something. (German has a similar way of phrasing the progressive. Anyone who knows German, think of bei(m) + verbal noun.) What I thought was neat about this in Gaelic is it reminded me of a dialect in English using what's called "a-prefixing" in folksy phrases like "Times sure are a-changin'." Lo and behold it does seem to be related and its usage and restrictions make a lot of sense in light of its roots as a preposition like we see here. Some interesting info I found: https://ygdp.yale.edu/phenomena/a-prefixing