In the parish of St. Dennis the church is dedicated to that saint. And when St. Dennis had his head cut off at Paris, blood, a legend says, fell on the stones of this churchyard; a similar occurrence often afterwards foretold other calamities.
From p31 in
Cornish Folk-Lore
M. A. Courtney
The Folk-Lore Journal > Vol. 5, No. 1 (1887), pp. 14-61
Saint Dennis, what with getting his head cut off, and then having the unusual ability to walk off somewhere carrying it, sounds very much a Celtic type of saint.
The wikipedia says that Denis was beheaded on the highest hill in Paris, which became known as Montmartre – the mountain of the martyr. The spot where he finished his headless wandering (whilst preaching a sermon) became the site of his shrine, and the eventual burial place of the kings of France.
en.wikipedia.org/wiki/Denis