Like his successor Ferchar, Máel Ísu is largely absent from the witness lists of Scottish royal charters, indicating a lack of involvement with the Franco-Gaelic Kings of Scotland. Máel Ísu, though, assisted with the invasion of the Kingdom of England by King David I.
Ailred of Rievaulx portrays Máel Ísu as the chief representative of the native Scottish nobility in their tension with the incoming French companions of the king. In a speech that Ailred may indeed have known about, Máel Ísu insists on the right of the Gaelic warriors to provide the van of the Scottish army. After the defeat of the Scottish army at the Battle of the Standard, Máel Ísu was required to give a son as a hostage
Máel Ísu's wife or wives are unknown to us. We do know, though, that he fathered Ferchar, who succeeded him to the Mormaerdom.