Attributing natural features and ancient constructions to kings, heroes and mythological beings is something that we find in the mythology, such as the Giants' Causeway, Black Pig's Dyke, the five principle routes etc. It's understandable that in the past people attempted to make sense of the world around them by concocting stories, many of which served the dual purpose of glorifying particular families through their ancestry.
Ringforts have had this treatment through the belief that they are "fairy forts" and linked to the Tuatha De Danann. But if we take a moment to actually read the sources or assess the results of excavations we find that ringforts were used right up to the 17th century where Gaelic influence still prevailed. Far from being mystical, ringforts are nothing more than medieval farmsteads. Some are small and quite mundane, while others are stunning examples of multi-vallation and would have been the residence of someone of wealth and importance.
The evidence is in the records and lying in the ground so why does this belief survive? Thankful as I am that it is this superstition which has protected archaeology from damage for some time it is still counter to what archaeologists are trying to achieve in their study of the past. We are trying to reconstruct the past and understand the lives of the people in it. To deify and glorify our predecessors makes it impossible to come to objective and logical conclusions about their lives.
All I can say is that those people who persevere in this belief must be lacking in the information which proves ringforts to be simple and mundane (though at the same time still fascinating if like me you're into that sort of thing) constructions. This information is not that hard to find, nor even to understand, so it makes me think that there are people out there who will continue to believe in quacky medicine, seances, horoscopes etc. If you believe the first things you learn and never set out to question it, or lack the facilities to understand work which proves it wrong, then that's that really. It's impossible to reach everyone, but as far as superstition is concerned, this one isn't really hurting anyone...