By the way, Celts had discovered Iceland long before Vikings (already in Ancient times), and they regularly visited it, but did not settle in large numbers because the climate was (at that time) too cold. However, there existed hermitages of Irish monks in Iceland, and such hermitages (already abandoned) were also found by first Viking settlers. It is also possible - but this issue is controversial - that there had existed some Celtic settlements, with some families (not just hermit monks) in Iceland already before first Vikings came.
Here is what one Polish biologist and amateur historian Grzegorz Jagodziński wrote about that (translation):
"In 330 BC Greek mathematician, astronomer and geographer Pytheas of Massalia reached the coasts of Scotland, and local inhabitants informed him about the existence of a land in the north, that he later called Thule (it was most probably Iceland).
Until year ca. 400 AD climate was too harsh to allow settlement in Iceland, but despite that the island was being regularly visited by British sailors - it is proven by discoveries of coins from that period. Most likely summertime fishing and hunting expeditions were taking place there. It is probable, that some ships reached also the coasts of Greenland.
Plutarch (ca. 40 - 120 AD) informs us about the existence of an island located far away to the west of Great Britain, bearing a name similar to the name of titan Kronos from Greek myths. The Sea of Kronos is how later the basin between Iceland and Greenland was referred too. "Inhabitants of Kronland" are mentioned in a document from 834 AD. The name "Greenland" was invented only later by the Vikings, in order to attract settlers (it was a typical gimmick!).
After 400 AD climate started to gradually get warmer, encouraging Pictish and Celtic inhabitants of the British Isles, called by the Vikings "people of the West" (Vestmenn), to organize regular sea cruises to Iceland, aimed at finding exotic resources, such as eiderdown, which were later being exported to the Mediterranean world. For sea travels Picts and Britons were using wooden ships, while Irish people were using currachs, covered by bovine skins. Such ships could transport up to 20 people, they were propelled by sails and oars.
Maritime activity of Picts and Celts lasted until the 7th century AD. They maintained regular commercial contacts with France, Norway and the Baltic countries. From that period originated
the legend about the the Voyage of Saint Brendan [recorded ca. 900 AD], who supposedly reached Iceland, Greenland, the island of Jan Mayen and maybe even the coast of America.
In some written records Greenland was referred to as Albania, which was Latin equivalent of Norse term Hvitramannaland ("White Men's Land"), or of Norse term Irland Mikkla ("Great Ireland") – in that latter case, it could be a name of a Celtic settlement in Greenland.
All of this proves that Iceland and Greenland were well-known, maybe even settled, already in period preceeding Viking voyages."