To expand on u/theimis' post, according to the 12th century Íslendingabók, before the Norse settlement of Iceland, there were Irish monks known as papar. This has been a matter of much scholarly debate, but some place names in Iceland have elements related to the word papar, so it is possible. Additionally, recent archaeological examinations by Kristján Ahronsson in the southwest of the country have found a cave with carved crosses that he claims are in a style consistent with Irish monks from around 800 (70 years before Norse settlement). I've seen these caves personally, and it was very interesting, but the crosses are very hard to date off of typology, so his claims are not universally accepted.

Regarding Greenland, the Dorset people inhabited Western Greenland up until around the time of the Norse settlement c. 1000, when they were replaced by the Thule people, whose descendants are the modern Inuit and Greenlanders.

Einar Ólafur Sveinsson, "Papar", Skírnir 119 (1945), pp. 170-203

Kristján Ahronsson, Into the Ocean:Vikings, Irish, and Environmental Change in Iceland and the North, 2015

Robert McGhee, The Last Imaginary Place: A Human History of the Arctic World, 2005