Goodwin's Culdic monk (Great Ireland) theory for the New England stone chamber sites strikes me as being the most likely. Read his book "The Ruins of Great Ireland in New England".

A most important point regarding these structures is that they all dowse. By this I mean that all of the stone chambers that I have visited have been built directly over a 90 degree intersection of water lines. If you can dowse, or know a dowser, go and check this out and you will find that it is so. Whoever built these structures, for whatever reason, saw such intersections as being a necessary factor in deciding their specific location. Dowsing is a very ancient tradition not shared (to my knowledge) by any of the Pre-Columbian or Amer-Indian peoples - yet one taken advantage of in the siting of early churches and cathedrals in Great Britain and France.