For "white robes" we should assume whitish robes as pure white as we known it was impossible to obtain before the advent of modern dye. The robes would have been of bleached wool and "how did they keep them clean?" - they washed them.

I have no conviction as to the truth behind this yarn, but its persistence makes it interesting. I included it on this thread because we were looking at the possibility of pre-Colunbam trans-Atlantic voyages. I see nothing impossible in the idea of Irish monks crossing the Atlantic in curraghs. Not directly maybe, but certainly via Iceland, Greenland, Baffin Island, Newfoundland and Nova Scotia - exactly the route that Leif Ericsson took.

I repeat the quotes I gave earlier in case you haven't seen them:

From Eirik's Saga, two skraeling boys tell Thorfin Karlsefni's men that " There was a country across from their own land where the people went about in white clothing and uttered loud cries and carried poles with patches of cloth attached. This is thought to have been Hvitramannaland."

Now that could be just medieval chroniclers' invention, but perhaps not. It does sound an awful lot like how natives might perceive chanting monks carrying religious banners.

Hvitramannaland means literally "White Men's Land" and in Landnamabok there is a reference to Hvitramannaland which was said "to lie six days' sail west of Ireland". Magnus Magnusson writes that "there may well be a connexion between this reference and the Tir na bhFear bhFionn (Land of the White Men) of Irish legend. Another Norse saga - Haukbok- names this western land as "Greater Ireland""

Something of substance there surely?