Obverse of a penny minted for Olaf at York showing a bird design. The legend reads ANLAF CVNVNGI (King Olaf).
Olaf III Guthfrithson (Óláfr Guðrøðarson; OIrAmlaíb mac Gofraidh), (died 941), a member of the Norse-GaelUí Ímair dynasty, was king of Dublin from 934 to 941. Gothfrith, his father, held both Dublin and York until Athelstan of England expelled him from York in 927.
After Athelstan's death in 939, Olaf again invaded York the same year, forcing Athelstan's successor, Edmund, into a treaty which ceded to Olaf Northumbria and part of Mercia. He did not get to enjoy his new lands for long, dying just two years later in 941. He was succeeded by Olaf Cuaran.
Under Olaf, Jórvík over-reached its own capacity of self-government. Despite the most extensive size and power wielded during his reign, the territorial framework of his kingdom was in sharp decline.
References
The Columbia Encyclopedia, Sixth Edition. 2001. [1]