From the longer Wikipedia page 
Northern Ireland is divided into six former local government divisions called counties, namely: Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone. Six largely rural administrative counties based on these were among the eight primary local government areas of Northern Ireland from its 1921 creation until 1973. The other two local government areas were the urban county boroughs of Derry (until 1899 part of county Londonderry) and Belfast (until 1899 split between counties Antrim and Down).
The six counties date from the Kingdom of Ireland; five were created between 1570 and 1591 in the Tudor conquest of Ireland, while county Londonderry dates from 1613 and the Plantation of Ulster. The total number of counties in the island of Ireland is 32, with Northern Ireland and the Republic of Ireland often respectively called "the Six Counties" and "the Twenty-Six Counties", especially by Irish nationalists opposed to the partition of Ireland. The 1898–1973 administrative counties were subdivided into county districts. The two-tier county/district system was replaced with a single-tier of "districts", numbering 26 in 1973 and rationalised into 11 in 2015. The areas corresponding to the eight counties and county boroughs remain in use for some administrative purposes, and the six counties retain a measure of popular identity.