High King of Ireland

There are two titles given to kings of Ireland: King of Tara, and High King of Ireland. These are not the same thing. "High King" was first used in the 9th century, although it was sometimes applied retroactively and anachronistically to figures of legend; "King of Tara" was first used in the 6th century, although retroactively applied to previous kings as well as legendary figures.

The Kingship of Tara is by far the older of the two, and does not necessarily denote ruling all of Ireland. It is associated with the Hill of Tara, a site that has been important since Neolithic times, with several Neolithic features including a passage tomb dated to 3200BCE, and a standing stone called the "Stone of Destiny" (brought to Ireland by the Tuatha dé Danann).

Possessing the Hill of Tara by conquering whichever tribe held it was a necessary step to claim this special kingship. In the 3rd century, the Laigin seized it from the Érainn; Niall of the Nine Hostages took it from the Laigin in the 5th century, after which it was possessed by the Ui Néill clan. 

The Hill of Tara is also associated with the title "High King." It is considered the place from which the High King rules, thanks to its legendary status. The High King of Ireland was also known as the "King of all Ireland," because unlike the King of Tara, the High King was one who united all the various kingdoms under one rule. Actually, "united" is probably too strong a word. The High King received tribute from the smaller kings, but did not directly rule their kingdoms. (With the rise of political and financial power in cities such as Dublin, Limerick, and Waterford, possessing those became more important after the 11th century.)

The King of Tara was a sacred title, and he "married the land" by having a marriage or a sexual relationship with a "sovereignty goddess" (a term found only in Celtic studies), Maeve. Gerald of Wales wrote that the would-be king sexually embraced a white mare, which was then slaughtered for a feast. Which brings us to Diarmait mac Cerbaill.

Diarmait mac Cerbaill is considered the last King of Tara to be part of the pagan ritual. Diarmait, however, also turned towards Christianity during his reign. I'll go into this dichotomy next time.