He was born around 850 AD and was the son of Olaf the White, King of Dublin, and Aud the Deep-minded, who was the daughter of Ketil Flatnose. After the death of Olaf, Aud and Thorstein went to live in the Hebrides, then under Ketil's rule.
Thorstein eventually became a warlord and allied with the Jarl of Orkney, Sigurd Eysteinsson. Together Thorstein and Sigurd waged a series of campaigns in Caithness, Sutherland, Ross, Moray, and a number of other regions, eventually receiving tribute from half of Scotland. However, the Scottish chieftains plotted against Thorstein, and he was killed; the exact nature of his death is unknown but it probably took place around 880 or 890. After Thorstein's death Aud left Caithness, sojourning for a while in Orkney before settling with other members of her clan in Iceland.