First we should correct the question. When they arrived the Norse did not encounter any natives in Greenland. They were still further up north in the arctic and arrived later.

As for Iceland, it is an open possibility that before the Norse came there, there had been a small population of Irish monks and possibly some other celtic population. This has not been confirmed by archealogical discoveries. The main sources are 12th century (much later) Icelandic writings but also written sources from Ireland that strongly suggest that they had dwelled in Iceland prior to the settlement.

Those would for example be the Monk Dicuilus (De Mensure Orbis Terrae, AD 825), the Priest Beda (AD early 700s) which is mentioned as a source in the Book of Settlements.