Before seeing these recent posts, I'd only known "geas" as a word from Celtic culture, which I'd understood as equivalent to a taboo (came across it, with that sense implied, in S.M. Stirling's "Emberverse" novel series, before I got fed up with said series -- the novels take place, partially in a strongly Celtic milieu). Was unaware that it could also be a compulsion to do something. Looking up the online Oxford Dictionary, I found: "geas -- in Irish folklore -- an obligation or prohibition magically imposed on a person".
Highly educational, this eHell !