Lundy, Isle of Avalon

Annwn 

The same author adds;-

'Annwn. The most famous of the Otherworld places which passed into Arthurian legend from Christian mythology. ***** 'It is an ISLAND, a realm of magic and of mysteries older than Christianity. ****** -Geoffrey Ashe in 'Guidebook to Arthurian Britain.'

'Annwyn could be entered via Lundy'- Geoffrey Ashe, Avalonian Quest.

"The island of Gwair's imprisonment 'Ynys Weir'  was also called Annwyn. It was also called 'Kaer Sidi.' There is a remarkable similarity between 'Kaer Sidi' and the 'isle of immortal elders' in 'Perlesvaus' also the island in the 'Navigation of St. Brendan.'  The fortress becomes the abbey which becomes the pagan temple of Joseph which becomes the Grail castle. Depending on the preconceptions of the teller and the listeners.

'Annwn is but one name for the isle where Gwair was imprisoned. Another name was Kaer Sidi. Anwyl suggested that this was the name for the castle where Pryderi of the golden hair was imprisoned.....remarkable similarity of Kaer Sidi to the isle of the immortal elders in Perlesvaus justifies us assuming that the mysterious captive in the pit was Gwair. Perceval goes on to another island where he liberates a chained youth, Galobrun." from 'Celtic Myth and Arthurian Romance'

The use of the name 'Avalon' - 'Insula Avallonis' - for the Celtic Otherworld probably predates Geoffrey of Monmouth's writing.  'Insula Avallonis' is used by Geoffrey of Monmouth as the equivalent of 'insula pomorum.' 

He writes of Arthur - 'Passing away after his last battle to the enchanted ISLE of Avalon.' - 'Insula Avalloni.' He also states that Arthur's enchanted sword, Excalibur, was 'forged on the ISLE of Avalon.'

 In the 'Vitae Merlini' by Geoffrey of Monmouth, 'Morgen' is named as the chief among nine sisters who rule over the 'insula pomorum que fortunata vocateur' to whom Arthur is brought after the battle of Camlann.