a short excerpt from an essay by Simon J. Cook:
Going up to Oxford in 1911, Tolkien as an undergraduate probably attended Rhys’ lectures; later, in a short essay of 1932, we find him engaging carefully with his scholarship. And it seems that Tolkien had read the Professor of Celtic’s 1900 presidential address. At one point in this lecture Rhys discusses certain ‘underground – or partially underground – habitations’ that, he believed, had been home to Britain’s natives. These abodes, he explains: appear from the outside like hillocks covered with grass, so as presumably not to attract attention... But one of the most remarkable things about them is the fact that the cells or apartments into which they are divided are frequently so small that their inmates must have been of very short stature.
But if Tolkien first stumbled upon a hobbit hole whilst reading Rhys’ lecture, it seems likely that his imagination drew also upon Buchan’s depiction of Britain’s natives..read the whole thing.
Buchan's book can be found HERE.
Greenwell's book on British Barrows is HERE