One of the Imramma, or 'Voyage' stories, dating to possibly the 8th century. I can only find a small portion of the text in Irish to translate but it is an interesting section and offers insight into how far back the concept existed that to live on one of the Otherworldly islands meant to be unable to return to our world without immediately dying should you touch the earth.
Imram Brian Meic Ferbail
[gap: chasm in MS/extent: uncertain]Lil in chertle dia dernaid boí in snáthe inna certle hi lláim inna mná consreng dochom poirt. Lotar iarom hi tegdais máir arránic imda cach lánamna and .i. tri .ix. n-imdae. In praind dobreth for cach méis nír ircran díib
Ba bliadain donarfás dóib bith and ecmaing batir ilbliadna ni tesbi cach mblas. Gabais eólcaire fer diib .i. Nechtan mac Collbrain. Atchid a cenel fri Bran ara tía leis dochom nErend. Asbert in ben ropad aithrech in fáboll dálotar cammae. & asbert in ben arná tuinsed nech díb a tír & ara taidlitís léu in fer fodnácaibset i n-Inis na Mell tar essi a chéli.
Dollotár iarom co tornachtatár in dáil hi Srúib Brain. Iarmofochtatarside dóib cía dolluid in muir. Asbert Bran messe or se Bran mac Febail. Ni beram achni aní sin ol a chéli di híu. Atá i ssenchassaib lenni chena Imram Brain.
Docurethar úadib in fer asin churuch. Amal condránicside fri talmannaib na Herend bá lúathred fó chetóir amal bid hi talom no beth tríasna hilcheta bliadna. Is and cáchain Bran in rand so.
Do mac Colbrain ba mór mbaíss
tórgud a láme fri aís
can nech dorratad toind usci glain
tar Nechtan mac Collbrain.
Adfet iar sin Bran a imthechta uli o thossuch co tici sin do lucht ind airechtais & scribais inn rundnu so tre ogum & celebrais doib iar sin & ní fessa a imthechta ónd úair sin.
- Lebor na hUidre
Wandering of Bran son of Ferbal
[begins after gap in text]
The white ball of thread was in his palm, the thread of the ball of thread was in the woman's hand pulling them towards port. Then they went into a large house where they found a bed for every couple there that is three times nine beds. The meal given on every platter was not emptied from them.
There was a year's growth to them in that world there but it happened that there were many years with no savour lacking to them. A longing for home took a man of them that is Nechtan son of Collbran. Bran saw his kindred against him that he should go with him to Ireland. The woman said an assault of regret would be the going nevertheless. And the woman said therefore none of them should touch the land and that they visit with the man left in Inis na Mell [island of delight] who they'd left out of their companions.
They went afterwards until they came to a gathering at Srúib Brain [Bran's headland]. The men asked who it was that had come across the sea.
Bran said "It is me, Bran son of Ferbal."
"We do not recognize this one" the other person said. "[but] There are old stories of Bran's Voyage."
He puts himself out, the man [Nechtan], from the coracle. So that as soon as he was against the earth of Ireland he was dust immediately as if he'd been under the earth without life through many years. Then chanted Bran the section following:
"To the son of Colbrain, was a lack of judgment
Bringing his hand against a lifetime
without conferring any wave of pure water
to Nechtan son of Collbrain."
Then Bran told of all his wanderings from the start until then to the people gathered there and wrote these sections in Ogham and told them farewell and no one knows his wanderings from that hour.