Early Irish monks were quite the adventurers. Besides building a monastery on remote locations such as Skellig Michael, they traveled to distant countries, and may have even reached North America before the Vikings and Columbus ever did.
The most famous explorer was a monk known now as St. Brendan and he was born in County Kerry, not too far from Skellig Michael (he was likely very familiar with the island). It’s even possible that the founder of the monastery at Skellig Michael (St. Finnian) was a member of St. Brendan’s crew and could help explain why they established a monastery on the distant isle.
In any case, the possibility of an early voyage to America is not out of question. Here is an excerpt of an article I wrote for Aleteia.org on the topic:
Later on [St. Brendan] heard of rumors of a distant land in the West which some believed was the original Garden of Eden. This gave Brendan all the motivation he needed and he was determined to discover where this Paradise on earth was located and preach the Gospel to any unbelievers he met along the way.
Taking a small crew, Brendan set out around the year 545 on a small round-bottom boat called a currach that was sealed with leather and had a square sail. An 8th-century account of the journey was recorded in The Voyage of St. Brendan and it describes various stops along the way that appear to correspond to places such as the Faroe Islands, Iceland, and Greenland, and even describes icebergs.
The possibility of such a feat was researched in 1976 by historian Tim Severin, who crafted an identical boat from the description given and took along with him a few companions. He made stops similar to those he believed St. Brendan could have made and eventually reached Peckford Island, Newfoundland. Severin’s research proved that a cross-Atlantic voyage was possible during the 6th century using primitive technology.