Visions like that are inexpicable. So I won't even offer opinion, let alone explanation.

A great many "forts" in Ireland are built near to or around cairns or burial mounds.

You raise a good question (or was it Nigel?) ... did they build the hillfort to protect these monuments? Maybe they did. I have always wondered why Europe's largest hillfort, above Baltinglass in Co. Wicklow, didn't use up the cairn material from the great triple passage tomb on the peak of the hill. The fort is 3000 years newer than the tombs though, so only some form of respect or fear could explain it.

Iron age secondary buials in Neolithic tombs (very common in Ireland) is perhaps an indication of tradition and respect. Though it can't be reverence, because if it was, then to put an invasive burial into it could be seen as sacrilage.

I think that a lot of the rath/ringfort associations with standing stones and mounds was for the prestige factor. A curious one is Ballymacgibbon cairn amongst others.

The Hill Of Tara, Navan Fort, Dun Ailine and Magh Adhair all have the 'enclosure around the mound' feature. These are all Royal Inauguration sites.

I don't think the protecting ancient sites is a valid option though. I think it's coincidence, pure and simple. The passage tomb builders built sites in remote, hard to get to, landscape dominating positions ... ideal locations for a hillforts. And yet a great number of such tombs do not have later hillforts around them.