It was gradually accepted during the 20th century that the Americas were discovered by the Vikings; but it seems likely that before this date, Irish monks had made the journey, taking the same route. Here (in a ten minute video) Caleb Howells lucidly summarizes the main evidence (which I found convincing, as I stated in the comments!):
Yet knowledge of the existence of the Americas was apparently forgotten in Europe by the Renaissance - or, if not forgotten, then withheld or suppressed.
Rudolf Steiner made some interesting remarks on this matter in 1917:
A particularly good home for spiritual life, protected against all possible illusions, was Ireland, the island of Ireland, in the first Christian centuries. More than any other spot on earth it was sheltered from illusions; and that is why so many missionaries of Christianity went out from Ireland in those early times.
But these missionaries had to have regard for the simple folk among whom they worked — for the peoples of Europe were very simple in those days — and also to understand the great impulses behind human evolution. During the fourth and fifth centuries Irish initiates were at work in central Europe and they set themselves to prepare for the demands of the future. They were in a certain way under the influence of the initiate-knowledge that in the fifteenth century — in 1413, as you know — the fifth post-Atlantean epoch [i.e. the 'modern' era - BGC] was to begin.
Hence they knew that they had to prepare for a quite new epoch, and at the same time to protect a simple-minded people. What did they do in order to keep the simple people of Europe sheltered and enclosed, so that certain harmful influences could not reach them? The course of events was guided, from well-instructed and honourable sources, in such a way that gradually all the voyages which had formerly been made from Northern lands to America were brought to an end.
Whereas in earlier times ships had sailed to America from Norway for certain purposes (I will say more of this to-morrow), it was gradually arranged that America should be forgotten and the connection lost. By the fifteenth century, indeed, the peoples of Europe knew nothing of America. Especially from Rome was this change brought about, because European humanity had to be shielded from American influences.
A leading part in it was played by Irish monks, who as Irish initiates were engaged in the Christianising of Europe. In earlier times quite definite impulses had been brought from America, but in the period when the fifth post-Atlantean [modern] epoch was beginning it was necessary that the peoples of Europe should be uninfluenced by America — should know nothing of it and should live in the belief that there was no such country.
Only when the fifth post-Atlantean [modern] epoch had begun, was America again “discovered,” as history says. But, as you know very well, much of the history taught in schools is fable convenue [false consensus], and one of these fables is that America was discovered for the first time in 1492.
In fact, it was only re-discovered. The connection had been blotted-out for a period, as destiny required.
Steiner seems not to have realized that Irish monks had themselves visited America; but does suggest that they knew of it and deliberately did not record or disseminate their stunning information about another continent to the laity.
Steiner believes that this decision was made for good reasons; because it was spiritually-desirable (a matter of 'destiny') that Europe develop separately for another millennium before being exposed to the revolutionary knowledge of the New World.
As I say, interesting...