Ah My Good Chicar -
thanks for the topic!
Hy-Brasil or ""Bracile" " is a "phantom island" west of Ireland. The Irish claimed it could not be found because it was constantly cloaked in mist
except for one day every seven years, when it becomes visible.... but still could not be reached.
It is similar to the various mythical villages that are "out of sync with time" except for "every so often...
like "Brigadoon" - yes the Scots village about which they made a musical AND a musical movie ....
The myth/legend is so prevelant one has to wonder after the origins.
there is also
Inis Oírr or Inisheer - a real island! the smallest and most eastern of the three Aran Islands in Galway Bay, Ireland.
it's claim to fame is that Inisheer is one of the finest examples of a Glacio-Karst (ice age) landscape in the world
Tech Duinn, a mythic island west of Ireland where souls go after death.
Great Ireland, a similarly west-of-Ireland place, Irish myths of which are believed to have influenced the Vikings.
these may be based on the REALLY OLD myths of the Tuatha De Danann - the "Old Ones" (some called them "gods" )
of Gaelic Ireland, who came from Tir na nOg - the island paradise west of Ireland.
yhs
prof marvel
the emerald Isles have an interesting history with much mystery. I will be investigating those places you mention further.