Since the early 17th-century there have been four Provinces of Ireland: Connacht, Leinster, Munster and Ulster. The Irish word for this territorial division, cúige, meaning "fifth part", indicates that there were once five, however in the medieval period there were more. The number of provinces and their delimitation fluctuated until 1610 when they were permanently set by the English administration of James I. The provinces of Ireland no longer serve administrative or political purposes, but function as historical and cultural entities.
In modern Irish the word for province is cúige (pl. cúigeadh). The modern Irish term derives from the Old Irish cóiced (pl. cóiceda) which literally meant "a fifth". This term appears in 8th century law texts such as Miadslechta and in the legendary tales of the Ulster Cycle where it refers to the five kingdoms of the "Pentarchy". In the 12th-century Lebor na Cert (Book of Rights), the term means province, seemingly having lost its fractional meaning with seven cúigeadh listed. Similarly this seems to be the case in regards to titles with the Annals of Ulster using the term rex in Chóicid (king of the fifth/province) for certain overkings.
The origins of the provinces of Ireland can be traced to the medieval cóiceda (literally "fifths") or "over-kingdoms" of Ireland. There were theoretically five such over-kingdoms, however in reality during the historical period there were always more. At the start of the 9th-century the following are listed: Airgíalla, Connachta, Laigin, Northern Uí Néill (Ailech), Southern Uí Néill (Mide), Mumu, and Ulaid. These seven over-kingdoms are again listed in the 12th-century Lebor na Cert.
Each over-kingdom was divided into smaller territorial units, the definition of which, whilst not consistent in Irish law tracts, followed a pattern of different grades. In theory in the early medieval period:
This pyramid structure however by the later medieval period had little validity.
Paul MacCotter proposes the following structure of lordship in the 12th-century: High-king of Ireland; semi-provincial king, such as Connacht, Ulaid, Desmumu; regional king, such as Dál Fiatach and Uí Fhiachrach Aidni; local king or king of a trícha cét, such as Leth Cathail or Cenél Guaire; and taísig túaithe at the bottom.
The kingdom of Osraige, which had its genealogy traced back by early Irish genealogists to the Laigin, was part of Mumu from the 6th to 8th century and ruled by the Corcu Loígde dynasty. By the 7th-century Osraige had lost their dependence on the Corcu Loígde, with the restoration of the local Dál Birn dynasty. Osraige remained part of Mumu until 859 when Máel Sechnaill I, king of the Uí Néill, forced Mumu to surrender it to his overlordship. After this situation ended it became an independent kingdom which gradually moved towards the Laigin sphere of influence as they sought to claim the Laigin kingship. It was during the 9th-century that Osraige, ruled by Cerball mac Dúnlainge, became a major political player.