That is indeed what I was saying when I uttered this:

"If you find a Roman coin at a site it just tells you that it was still sacred during the first four hundred years CE."

Meaning that it couldn't be a Norman construction - although the Norman's could have altered or added to it. I've mentioned this before: a few mottes in Ireland have been excavated and found to have burials at the base. They were done a long time ago, so there is no good record of whether the mottes were built on barrows or if the bodies were put there as some sort of offering. Knowing the Norman Xtianity side of things I think it's more likely that the Normans came and said something along the lines of, "We need a motte here. Ooh Look a mound we can use as a quick start!", but in French.