Eystein was born, apparently in Scotland, the son of Harald Gille, who was king of Norway from 1130 to 1136, and a woman named Bjaðǫk.
Eystein first appears in the sagas in 1142, when several Norwegian lendmenn travelled west and fetched him back to Norway from Scotland.
He then proceeded to loot along the Scottish and English coast, attacking Aberdeen, Hartlepool and Whitby, in a voyage reminiscent of the earlier viking expeditions.
According to the sagas, Eystein and Sigurd had plotted to strip Inge of his royal title and divide his share of the kingdom between them.
His bastard son Eystein Meyla was proclaimed king by the Birkebeiner party in 1176, but was defeated and killed the year after.
The main sources to Eystein's reign are the kings' sagas Heimskringla, Fagrskinna, Morkinskinna and Ágrip.
The three former base at least part of their account on the older saga Hryggjarstykki, which was written some time between 1150 and 1170, and was thus a near-contemporary source.