Gweledigaethau y Bardd Cwsg (Visions of the Sleeping Bard)
Abercywarch is trumpeted by George Borrow in his usual gushing prose "to be the place where Ellis Wynn [sic] composed his immortal Sleeping Bard, the book which I translated in the blessed days of my youth composed amidst the wild and wonderful scenes which I here behold"
I'd never heard of the guy, so I did a bit of digging around, and came up with the following:
Ellis Wynne (1671-1734), of Lasynys Fawr near Harlech, was a "cleric and satirist, one of the most stylish and dynamic prose writers in the Welsh language", and he believed that Great Britain was a stronghold of faith in an evil world. (Oh dear - he must be turning in his grave.)
His book "Gweledigaethau y Bardd Cwsg" or "Visions of the Sleeping Bard" was published in 1703 by the Society for the Propagation of Christian Knowledge (SPCK). Its main theme is the improvement of morals, and it's based upon an original Spanish satire. Cue George Borrow again, in another of his books:
"he was carried away in his sleep by [fairies] over mountains and valleys, rivers and great waters, incurring mighty perils at their hands, till he was rescued from them by an angel of the Most High, who subsequently showed him many wonderful things. The angel showed him the course of this world, its pomps and vanities, its cruelty and its pride, its crimes and deceits. On another occasion, the angel showed him Death in his nether palace, surrounded by his grisly ministers, and by those who are continually falling victims to his power. And, on a third occasion, the state of the condemned in their place of everlasting torment".
(He continues in this manner for some time, but I'm sure you get the general drift of things.)
However, if you want to know more, read "Y Bardd Cwsg a'i Gefndir" (The Sleeping Bard and his Background), a study of Ellis Wynne written by Gwyn Thomas, professor of Welsh at Bangor University, and published in 1971.