Michael Quinion makes a compelling argument in agreement with Jack:http://www.worldwidewords.org/topicalwords/tw-sha1.htm
As regards the word meaning a shelter, apparently it was first recorded in Ohio, and shortly after that in Canada, prior
to the great influx of immigrant Irish workers. Either way, dated recorded evidence doesn't support an Irish origin for it even though the Irish "old house" meaning fits the sound of it so well.
I too grew up with the folk etymology that "shanty" came directly from the Irish.