Iron Age wall ? It's impossible trying to figure it out. Clearly there was little border between the sacred and the profane. Here's my daft one - there are three layers but - http://www.themodernantiquarian.com/post/40654 .

The wall in the case of Millin Bay was there before the chambered tomb was built as far as I recall, there's a bit about it in one of Burl's books. Beaghmore was another complex built on top of pre-existing neolithic field walls that were left as they were in a lot of places.