The History of the Lordship of the Isle is long and complex, so for the sake of brevity and staying on topic I will limit the scope of this entry to a basic understanding how the title Lord the Isles came to be part of the titles to the heir to the British throne.
HRH The Lord of the Isles
The Lord of the Isles (Scottish Gaelic: Triath nan Eilean or Rìgh Innse Gall) is a title of Scottish nobility with historical roots that go back beyond the Kingdom of Scotland. The Isles refers to west coast and islands off of present-day Scotland were those of a people or peoples of uncertain cultural affiliation lived until the 5th century.
The Isles emerged from a series of hybrid Viking/Gaelic rulers in the Middle Ages, who wielded sea-power with fleets of galleys (birlinns). Although they were, at times, nominal vassals of the Kings of Norway, Ireland, or Scotland, the island chiefs remained functionally independent for many centuries. Their territory included the Hebrides (Skye and Ross from 1438), Knoydart, Ardnamurchan, and the Kintyre peninsula. At their height they were the greatest landowners and most powerful lords in Britain after the Kings of England and Scotland. Though seen as a title of Scottish Nobility at one point the Lords of The Isles held the title of King along with sovereignty.
In 973, Maccus mac Arailt, King of the Isles, Kenneth III, King of the Scots, and Máel Coluim I of Strathclyde formed a defensive alliance, but subsequently the Scandinavians defeated Gilla Adomnáin of the Isles and expelled him to Ireland.
In the late 11th Century the Norse nobleman Godred Crovan became ruler of Man and the Isles, but he was deposed in 1095 by the new King of Norway, Magnus Bareleg. In 1098, Magnus entered into a treaty with King Edgar of Scotland, intended as a demarcation of their respective areas of authority. Magnus was confirmed in control of the Isles and Edgar of the mainland. Lavery cites a tale from the Orkneyinga saga, according to which King Malcolm III of Scotland offered Earl Magnus of Orkney all the islands off the west coast navigable with the rudder set. Magnus then allegedly had a skiff hauled across the neck of land at Tarbert, Loch Fyne with himself at the helm, thus including the Kintyre peninsula in the Isles’ sphere of influence. (The date given falls after the end of Malcolm’s reign in 1093.)
Clan Donald, also known as Clan MacDonald (Scottish Gaelic: Clann Dòmhnaill is a Highland Scottish clan and one of the largest Scottish clans. By the Thirteenth Century the Norse-Gaelic Clan Donald came to rule the independent Isles. The Norse-Gaelic Clan Donald traces its descent from Dòmhnall Mac Raghnuill (d. circa 1250), whose father Reginald or Ranald was styled “King of the Isles” and “Lord of Argyll and Kintyre”. Ranald’s father, Somerled was styled “King of the Hebrides”, and was killed campaigning against Malcolm IV of Scotland at the Battle of Renfrew in 1164. The chiefs of the Clan Donald held the title of Lord of the Isles until 1493 and two of those chiefs also held the title of Earl of Ross until 1476.
Successive Lords of the Isles fiercely asserted their independence from Scotland, acting as kings of their territories well into the 15th century. Then in 1462, John MacDonald II, Lord of the Isles, signed a treaty with Edward IV of England to conquer Scotland with him and the Earl of Douglas. The treaty between Edward IV and MacDonald II has been used to show how the MacDonald Lords were viewed as independent rulers of their kingdom, freely entering into national and military treaties with foreign governments.
King James IV of Scotland 1488-1513
Unfortunately for the MacDonald sovereigns, the civil war in England, known as the Wars of the Roses, prevented the completion of the alliance between Edward IV and MacDonald II. Upon the discovery of his alliance with Edward IV in 1493, MacDonald II had his ancestral lands, estates, and titles taken from him by James IV of Scotland. In addition to James IV seeking revenge on MacDonald II, he possessed a larger military force and was able to impose his will on the West Coast of Scotland, though uprisings and rebellions were common.
Though the Lordship was taken away from the MacDonald family in the 15th century, waves of successive MacDonald leaders have contested this and fought for its revival ever since. However, once the land and title where seized by the Scottish Crown the eldest male child of the reigning Scottish (and later, English then British) monarch has been styled “Lord of the Isles.” The office itself has been extinct since the 15th century and the style since then has no other meaning but to recall the Scottish seizure of the ancient Norse-Gaelic lordship and crown. Thus Prince Charles, The Prince of Wales, is the current Lord of the Isles.