From Alan James – 7 August 2020
I take it Roibeard’s wondering whether it’s related to Gaelic deal-tholl, as is said in the SND, from deala ‘a leech’ + toll, verb ‘to bore’, noun ‘a hole’. MacTaggart’s spelling towal makes it look like a disyllable, which I suppose raises doubt about Gaelic toll. But ‘gell’ is Gaelic geal, listed by Dwelly as a variant of deala.
MacTaggart’s explanation is gruesome: ‘The horse-leeches … continue sucking so long as they can get a drop of blood, while the life-stream flows out of their nether end, whence the name “Towals”, or tails, leeches at either end.’