Something else occurs to me that reinforces the opinion that none of the Irish portals were covered. Not one has ever been found in recent times beneath a mound, or even partially covered. It seems unlikely that all of 174 documented examples have had their original mounds removed by someone. If they were it had to be a someone, as rain would not do it.
Distribution. The majority above a line running from Mayo to Dublin, with 10 out of 15 in Galway and Mayo being on the coast. A cluster in a line running from Dublin to Waterford, and only 2 s.w of a line running Waterford/Limerick.