Amy's family was Clann Ruaidhrí, descendants of Ruaidhrí, a son of Somerled, the 12th-century warlord who had founded a dynasty on the west coast of Scotland by driving the Norse out of the Inner Hebrides. By Amy's time the Clann Ruaidhrí lands stretched across Garmoran on the mainland - roughly modern Moidart, Morar, Knoydart and Arisaig - and out into the North Isles, including the Uists and Barra. Her father was Ailéan mac Ruaidhri. Robert the Bruce, who had spent his fugitive year in 1306-7 sheltered by the Hebridean lords, was careful to keep their loyalty after he won the kingdom. Bruce confirmed Clann Ruaidhrí in its lands. But he gave Dunstaffnage Castle, the symbolic seat of Lorn, to a royal constable named Arthur Campbell, not to a member of the family. The crown wanted leverage on the western seaboard.

Around 1325, late in the reign of Robert the Bruce, Amy's brother Ruaidhri mac Ailein was dispossessed of his lands for engaging in plots against the king. The detail of what he had done is lost. What survives is the political reality: the western Gaelic lords were never quite at ease with a Scottish king they did not consider their feudal superior, and the Bruce's government was never quite confident that loyalty given freely would not be withdrawn. There was a third sibling, Ailéan, of whom almost nothing is known. Amy was now part of a family in royal disgrace, her brothers out of favour, her inheritance uncertain. She was still a great heiress in waiting, because dispossession of one generation in medieval Gaelic Scotland was rarely the end of the story.

Edward Balliol, briefly king of Scotland during the disputed succession that followed Bruce's death in 1329, may have restored these lands to Amy's brother Raghnall mac Ruaidhri. The grant was formalised by David II, Bruce's son, around 1344. Raghnall was now the holder of Garmoran and the North Isles, though Lorn was kept by the crown and Lochaber went to John of Islay, the rising power in the southern Hebrides. The arrangement did not last. In October 1346, Raghnall was assassinated at Elcho Nunnery near Perth, killed as the result of a quarrel with Uilleam III, Earl of Ross. Raghnall was, in the chronicler's phrase, the "last chieftain of the MacRuaris." His lands now passed to his only surviving sister: Amy.

The marriage of Amy and John of Islay had already taken place, sometime in the 1330s. With Raghnall's death Amy became the carrier of an enormous inheritance, and John of Islay - already styling himself Dominus Insularum from 1336 - now ruled, through his wife, a swathe of western Scotland that approached the territorial extent of the medieval Kingdom of Mann and the Isles. The title looked back to the old Norse-Gaelic kingdom that had once ruled the Hebrides as an independent power. The marriage produced sons. Amy had borne him at least three: John, Ranald and Godfrey. Through Ranald descended the Clanranald, one of the most powerful Hebridean kindreds of the late medieval period.

Around 1350 John annulled the marriage and married Margaret Stewart, daughter of the man who would become King Robert II of Scotland. The strategic reasoning was clear; the personal cost to Amy is not recorded, but it must have been profound. She had been the carrier of his lordship over half a coast, and now she was the discarded first wife. By tradition she retired to her own lands and lived out her later years on Garmoran. The castle of Tioram, the dry-island fortress on Loch Moidart, is associated with her name. Her sons by John were excluded from the main succession, which passed to her replacement's children, but Ranald's descendants - the Clanranald - held the Garmoran lands for centuries. She is remembered now mainly through them, and through the medieval records that survive almost by accident. A noblewoman in her own right, twice the carrier of an inheritance, and a person whose life was conducted at the highest political level of her time, was set aside when the politics changed and the chronicles closed over her.