By later Campbell tradition, Niall was the elder son of Cailean Mór; however, contemporary evidence seems to suggest that his brother Domhnall enjoyed this distinction.
Niall served in the warband of Richard Óg de Burgh, 2nd Earl of Ulster and in the "English" army which besieged Stirling Castle in 1305.
[6] In the same year, Niall and his brother Domhnall received the guardianship of the heiresses of Andrew de Crawford, lord of the Baronies of Loudoun, Lochmartnaham and Draffan.
When Robert de Bruce decided to raise the Scottish banner in 1306, it is not surprising that Niall and Domhnall were among the would-be king's first adherents.
[8] All the evidence suggests that Niall remained in King Robert's warband for the years to come, fighting both the English-side generally and the MacDougalls in the west of Scotland.
Niall, however, had been married previously to Alyse de Crawford,[9] by whom he had at least two sons, Sir Colin Og Campbell of Lochawe and Dubhghall.
In 1315, King Robert granted the baronies of Loch Awe and Ardscotnish to Cailean for the service of a 40-oared galley for 40 days per annum.