|Wee_malky refers to the inside of the cairn possibly being unsafe. Certainly a consideration if you're sitting in there with the best part of 2m of cairn material above you!
There is also evidence of a repair job having been necessary during the building of the cairn or while it was still in use.
As you enter the cairn, you will see a 1.6m pillar with some cairn material to your right. The pillar supports the first lintel of the chamber (the one after the surviving passage lintel), the N end of which does not rest on a chamber stone in the way the other lintels do.
Presumably the lintel broke or (more likely) was threatening to and the pillar was put in to support it. The cairn material to its right could be further support or the blocking off of that area.
Erskine Beveridge in his 1911 North Uist , Its Archaeology and Topography (recently reprinted) says of this site,
We have been assured upon the best authority that Langass Barp contains a second chamber with its separate access from the north side, our informant having entered this within the past thirty years; while it is also stated that even a third chamber exists. Upon this subject we can add little, except that the east chamber already disclosed occupies but a small proportion of the whole structure, ample space remaining for at least two others. No Hebridean Passage Grave has produced definite evidence of more than one chamber and, because many are ruined, the plans of most are known. Perhaps the locals were just having a little fun.
Posted by greywether
21st September 2004ce
Edited 13th August 2008ce