From http://www.undiscoveredscotland.co.uk/usbiography/ghi/donaldofislay.html

Donald inherited his father's titles in 1387 and spent the early years of his tenure suppressing a revolt by his younger brother John Mór who, with the support of the Macleans, was trying to expand on his small inheritance of lands. By 1395 Donald had driven John Mór into exile in Ireland and beaten off the challenge of the Macleans.

___________________________________________________

From Wiki

Domhnall spent some of his first years as Lord of the Isles suppressing a revolt by his brother John Mór. John was Domhnall's younger brother, and resented his meagre inheritance. Although he was recognized as heir-apparent (tànaiste), he only received patches of land in Kintyre and Islay. The rebellion started in 1387 and went on into the 1390s, and John obtained the support of the MacLean kindred. However, John and the MacLeans were eventually forced to submit to Domhnall, and by 1395 John Mór had been forced into Ireland. There he entered the service of King Richard II of England and later established a MacDonald lordship in Antrim.