Little is known of Sir John beyond the genealogies of the Clan. He was a party to the Treaty of Ardthornish but is not heard from again for many years. There is every reason to believe that during this period he resided on his Irish estates.
The upheaval caused by the final forfeiture of the Lord of the Isles in 1493, however, brings him again into prominence. At that time King James visited the Highlands and John rendered homage. He was knighted by the King and confirmed in his old possessions; but the King left a garrison at the castles of Tarbert, Dunaverty and others which gave offence to the proud Sir John. Before the King had left Kintyre, Sir John and his son John Cathanach stormed Dunaverty, dislodged the lowland garrison and hanged the governor. On the King's return to Edinburgh, Sir John was declared a traitor and he was summoned for treason. The rebel knight ignored the summons and betook himself to Islay. Here he might have been safe were it not for the treachery of his kinsman John MacIan of Ardnamurchan, who in the guise of friendship apprehended Sir John and his son John Cathanach and their accomplices. After a summary trial, Sir John and his son were convicted and hanged on Broughmuir in 1499. John Cathanach's three sons, John Mor, John Og and Donald Balloch were executed at the same time. His remaining son, Alexander, fled to Ireland.
Sir John Mor had married Sarah, daughter of Felim O'Neil of Clanehoy by whom he had two sons, Hohn Cathanach and Alistair Carrach. The latter settled in Ireland. He was knighted by the Earl of Sussex in 1556 for services against the Irish and Scots and was granted the Barony of Dunbece.
John Cathanach, having been executed at the same time as his father, never served as Chief of Clan Iain Mhoir. Since the title to that position passes through him, he is named next in order. He married Cecelia Savage, daughter of Lord of the Ardes in Antrim, by whom, among others, he had Alexander Konnel, who succeeded his grandfather, Sir John Mhoir.