I think it's the same as Irish cottage knitting.
Same idea, anyhow.
A refinement of the idea is the knitting sheath or knitting belt that used to be common throughout Britain in the centuries BC (before circs). This is a leather pad stuffed with horsehair. It has holes into which you stick one long needle while working with the other.
Someone mentioned finding one at Schoolhouse Press a while back. You might also try Woodland Woolworks or RamWools, which have a lot of traditional knitting items. Or there is this:
I've also seen a needle holder for one-handed knitters at a website specializing in "ability" tools for people with disabilities. Again, the idea is to immobilize one needle so the knitter works with the other. You could probably rig something with a clamp or vise...
Finally, try posting on knittersreview.com A superb knitter on their forums mentioned that one of her hands doesn't work well and she used a brace to hold the needle in place. Hope you get a response. One way or another, we'll get your mother-in-law knitting again.