Ireland has historically been divided into four provinces, although the Irish-language word for this territorial division, cúige (literally: "fifth part"), indicates that there were once five - Meath (now incorporated into Leinster), being the fifth.

The four provinces are:

The origins of these provinces (loosely federated kingdoms with somewhat flexible boundaries) of which there were five in existence prior to the coming of the Normans can be traced to the overriding influence exerted in their respective territories by the great Irish dynastic families of O Neill (Ulster), O Melaghlin ( Mide), O Brien (Munster), O Conor (Connacht) and MacMurrough-Kavanagh (Leinster). 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 Irish Annals these five ancient political divisions were invariably referred to as Cúigí, i.e ‘fifth parts’, 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.

In modern times they have become associated with groups of specific counties, although they have no legal status. They are today seen in a sporting context, as Ireland's four professional rugby teams play under the names of the provinces, and the Gaelic Athletic Association has separate Provincial councils and Provincial championships.

The provinces were supplanted by the present system of counties after the Norman occupation in the twelfth century.

Six of the nine Ulster counties form modern-day Northern Ireland, which is part of the United Kingdom. Northern Ireland is often referred to as a province of the United Kingdom. These two inconsistent usages of the word "province" (along with the use of the term "Ulster" to describe Northern Ireland) can cause confusion.