High King of Ireland (Irish: Ardrí na hÉireann [ˈaːɾˠd̪ˠˌɾˠiː n̪ˠə ˈheːɾʲən̪ˠ]) was a royal title in Gaelic Ireland held by those who had, or who are claimed to have had, lordship over all of Ireland. The title was held by historical kings and was later sometimes assigned anachronously or to legendary figures.

Medieval and early modern Irish literature portrays an almost unbroken line of High Kings, ruling from the Hill of Tara over a hierarchy of lesser kings, stretching back thousands of years. Modern historians believe this scheme was crafted in the 8th century from the various genealogical traditions of powerful dynasties, and intended to justify their status by projecting it far into the past. John T. Koch explains: "Although the kingship of Tara was a special kingship whose occupants had aspirations towards supremacy among the kings of Ireland, in political terms it is unlikely that any king had sufficient authority to dominate the whole island before the 9th century".

The concept of national kingship is first articulated in the 7th century, but only became a political reality in the Viking Age, and even then not a consistent one. While the High Kings' degree of control varied, they never ruled Ireland as a politically unified state, as the High King was conceived of as an overlord exercising suzerainty over, and receiving tribute from, the independent kingdoms beneath him.

Early Irish kingship was sacred in character. In some early Irish sources, High Kings can gain their power through a marriage to, or sexual relationship with, a sovereignty goddess. The High King is free from blemish, enforces symbolic buada (prerogatives) and avoids symbolic geasa (taboos).

According to 7th- and 8th-century law tracts, a hierarchy of kingship and clientship progressed from the rí tuaithe (king of a single petty kingdom) through the ruiri (a rí who was overking of several petty kingdoms) to a rí ruirech (a rí who was a provincial overking). (See Rí.)

Each king ruled directly only within the bounds of his own petty kingdom and was responsible for ensuring good government by exercising fír flaithemon (rulers' truth). His responsibilities included convening its óenach (popular assembly), collecting taxes, building public works, external relations, defence, emergency legislation, law enforcement, and promulgating legal judgment.

The lands in a petty kingdom were held allodially by various fine (agnatic kingroups) of freemen. The king occupied the apex of a pyramid of clientship within the petty kingdom. This pyramid progressed from the unfree population at its base up to the heads of noble fine held in immediate clientship by the king. Thus the king was drawn from the dominant fine within the cenél (a wider kingroup encompassing the noble fine of the petty kingdom).

The kings of the Ulster Cycle are kings in this sacred sense, but it is clear that the old concept of kingship coexisted alongside Christianity for several generations. Diarmait mac Cerbaill, king of Tara in the middle of the 6th century, may have been the last king to have "married" the land. Diarmait died at the hands of Áed Dub mac Suibni; some accounts from the following century state that he died by the mythic Threefold death appropriate to a sacral king. Adomnán's Life tells how Saint Columba forecast the same death for Áed Dub. The same Threefold Death is said in a late poem to have befallen Diarmait's predecessor, Muirchertach macc Ercae, and even the usually reliable Annals of Ulster record Muirchertach's death by drowning in a vat of wine.