Cóiced

Ireland has historically been divided into four provinces: Connacht, Leinster, Munster and Ulster. The Irish word for this territorial division, cúige, literally meaning "fifth part", indicates that there were once five; the fifth province, Meath, was incorporated into Leinster, with parts going to Ulster. The provinces of Ireland serve no administrative or political purposes, but function as historical and cultural entities.

The origins of these provinces (loosely federated kingdoms with somewhat flexible boundaries), of which there were five before the Norman invasion, can be traced to the over-riding influence exerted in their respective territories by the great Irish dynastic families of Uí Néill/O'Neill (Ulster), Uí Máeilsheáchlainn/O'Melaghlin (Mide), Uí Briain/O'Brien (Munster), Uí Conchobhair/O'Conor (Connacht) and Mac Murchadha-Caomhánach/MacMurrough-Kavanagh (Leinster). A "king of over-kings", a rí ruirech was often a provincial (rí cóicid) or semi-provincial king to whom several ruiri were subordinate. Entities belonging to the 1st and 2nd millennia are listed. These do not all belong to the same periods. Over the centuries, the number of provincial kings varied between three and six. No more than six genuine rí ruirech were ever contemporary, with the average being three or four. Also, following the Norman invasion, the situation became somewhat more condensed and complicated than previously. The Norman invasion began in 1169, and the Normans went on to occupy Ireland until 1541.

In the post-Norman period the historic provinces of Leinster and Meath gradually merged, mainly due to the impact of the Pale, which straddled both, thereby forming the present-day province of Leinster. In the early Irish annals these five ancient political divisions were referred to as cúigí ("fifths") such as the fifth of Munster, the fifth of Ulster and so on. Later record-makers dubbed them provinces, in imitation of the Roman imperial provinciae.

...
Wikipedia