Rí in the context of "Máel Sechnaill mac Máele Ruanaid"

The Latin title rex has the meaning of "king, ruler" (monarch). It is derived from Proto-Indo-European *h₃rḗǵs. Its cognates include Sanskrit rājan, Gothic reiks, and Old Irish rí, etc. Its Greek equivalent is archon (ἄρχων), "leader, ruler, chieftain".

The chief magistrate of the Roman Kingdom was titled Rex Romae (King of Rome).

The Irish nobility could be described as including persons who do, or historically did, fall into one or more of the following categories of nobility:

• Gaelic nobility of Ireland: descendants in the male line of at least one historical grade of king (Rí).
• Hiberno-Norman or Old English (Ireland) nobility: descendants of the colonisers who came to Ireland from Wales, Normandy and England after the Norman invasions of England and Ireland in 1066 and 1169–71, respectively.
• Peerage of Ireland, whose titles were created by the English and later British monarchs of Ireland in their capacity as Lord or King of Ireland.

These groups are not mutually exclusive. There is some overlap between the first two groups (prior to the Treaty of Limerick), and a lesser degree of overlap between the last two groups (prior to independence from the United Kingdom). Such overlaps may be personal (e.g. a Gaelic noble who was "regranted" his titles by King Henry VIII of England), or they may be geographical (i.e. different noble traditions co-existing in neighbouring parts of the country, which were only distinguished by the date when they finally fell under the Dublin Castle administration).

Lord of the Isles or King of the Isles (Scottish Gaelic: Triath nan Eilean or Rìgh Innse Gall; Latin: Dominus Insularum) is a title of nobility in the Baronage of Scotland with historical roots that go back beyond the Kingdom of Scotland. It began with Somerled in the 12th century and thereafter the title was held by a series of his descendants, the Norse-Gaelic rulers of the Isle of Man and Argyll and the islands of Scotland in the Middle Ages. They 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 much of Argyll, the Isles of Arran, Bute, Islay, the Isle of Man, Hebrides (Skye and Ross from 1438), Knoydart, Ardnamurchan, and the Kintyre peninsula. At their height they were the greatest landowners and most powerful lords after the kings of England and Scotland.

The end of the MacDonald Lords came in 1493 when John MacDonald II had his ancestral homeland, estates, and titles seized by King James IV of Scotland. After that time, the MacDonald Clan contested the right of James IV to the Lordship of the Isles and uprisings and rebellions against the Scottish monarch were common. More recently, the Lordship of the Isles has been held by the Duke of Rothesay, the eldest son and heir apparent of the King of Scots, a title which, since the creation of the Kingdom of Great Britain, is usually borne by the Prince of Wales. Thus Prince William is the current Lord of the Isles.

The Kingdom of Breifne or Bréifne (Irish: [ˈbʲɾʲeːfʲnʲə]), anglicized as Breffny, was a medieval overkingdom in Gaelic Ireland. It comprised what is now County Leitrim, County Cavan and parts of neighbouring counties, and corresponds roughly to the Roman Catholic Diocese of Kilmore. It had emerged by the 10th century, as a confederation of túatha headed by an overking drawn from the Uí Briúin Bréifne.

By the 11th century, Bréifne was ruled by the Ua Ruairc (O'Rourke) dynasty. The kingdom reached the height of its power in the 12th century, under Tigernán Ua Ruairc. During the latter part of his reign, Bréifne took part in campaigns against the Norman invasion of Ireland. His assassination by the Anglo-Normans in 1172 was followed by a succession dispute, and a conflict between the Ua Ruairc and Ua Raghallaigh (O'Reilly) dynasties.

Ulaid (Old Irish, pronounced [ˈuləðʲ]) or Ulaidh (Modern Irish, pronounced [ˈʊlˠiː, ˈʊlˠə]) was a Gaelic over-kingdom in north-eastern Ireland during the Middle Ages made up of a confederation of dynastic groups. Alternative names include Ulidia, which is the Latin form of Ulaid, and in Cóiced, Irish for 'the Fifth'. The king of Ulaid was called the rí Ulad or rí in Chóicid.

Ulaid also refers to a people of early Ireland, and it is from them that the province of Ulster derives its name. Some of the dynasties in the over-kingdom claimed descent from the Ulaid, but others are cited as being of Cruithin descent. In historical documents, the term Ulaid was used to refer to the population group of which the Dál Fiatach was the ruling dynasty. As such, the title rí Ulad held two meanings: over-king of the Kingdom of Ulaid and king of the Ulaid people, as in the Dál Fiatach.