Antrim sits in the northeast corner of Northern Ireland, occupying 3,086 square kilometres along the northeastern shore of Lough Neagh. The county is the most populous in Northern Ireland - 651,321 people at the 2021 census - and the second most populous on the island of Ireland after County Dublin. Most of Belfast, the capital of Northern Ireland, is in Antrim, with the rest in County Down. A great deal of Antrim is hilly: Knocklayd, Slieveanorra, Trostan, Slemish (where St Patrick is said to have herded sheep), Agnew's Hill, and Divis. The inland slope is gentle but the northern coast falls in sheer basalt cliffs - the Antrim Plateau, formed in the Paleocene around 60 million years ago, is the same volcanic episode that built the Giant's Causeway, Fair Head, and the related formations of Staffa across the North Channel. The eastern coast is wider and softer - the Glens of Antrim drop down to a string of resort villages between Cushendun and Whitehead. Rathlin Island sits north of Ballycastle, basalt and limestone like the mainland.

In the early Middle Ages, northern Antrim was part of Dál Riada, a kingdom that stretched across the North Channel into what is now western Scotland. The shared sea was a road, not a barrier - Dál Riada is the reason Gaelic survived in both Ireland and the Scottish Highlands. South of Dál Riada lay Ulidia, ruled by the Dál Fiatach. Between them were the Dál nAraide and the Cruthin, pre-Gaelic Celts thought to be related to the Picts of Britain. Vikings raided the coast from the eighth century onwards. In the fifteenth century, the MacDonnells - known as Macdonalds in Scotland - became the most powerful of the gallowglass septs in the Glens. The Antrim coast looked east as much as south, and the Scottish connection is in the place names, the surnames, the cadences of Ulster Scots, and the basalt itself. In 1588, the Antrim coast was the scene of one of twenty-four Spanish Armada wrecks in Ireland. La Girona, off Lacana Point near the Causeway, was the worst of them.

Antrim has been Protestant-majority since the Plantation of Ulster in the seventeenth century brought Scottish and English settlers to the region. The 2021 census revealed that those of a Protestant and Other Christian community background were no longer a majority in Antrim, comprising 47.0 percent of the population, a sharp decline from 75.2 percent in 1861. The shift is most pronounced in Belfast, where Catholics are now a plurality in the city and its metropolitan area, but it is also a reflection of post-Troubles immigration from elsewhere in the world and of rising rates of irreligion. The county is split across six modern districts - Antrim and Newtownabbey, Belfast, Causeway Coast and Glens, Mid and East Antrim, Lisburn and Castlereagh, and parts of Armagh City, Banbridge and Craigavon. The traditional county town is Antrim. The seat of county government, more recently, was Ballymena.

Three things in Antrim attract the visitors who do not come for Belfast. The Glens of Antrim - nine narrow valleys running down to the eastern coast - hold some of the most isolated rugged landscapes in Ireland. The Giant's Causeway is a UNESCO World Heritage Site and the most famous geological feature on the island. Bushmills, two miles south of the Causeway, claims to be the world's oldest licensed distillery on the strength of a 1608 royal grant. Portrush, on the north coast, is a Victorian seaside resort that has twice this decade hosted the Open Championship at Royal Portrush Golf Club. Belfast International Airport - sometimes still called by its old name, Aldergrove - is the fifth-largest regional air cargo centre in the United Kingdom and the gateway for most visitors. The county is laced together by Translink Northern Ireland Railways: the Belfast-Antrim-Ballymena-Coleraine-Derry main line, the Belfast-Carrickfergus-Larne line to the ferry, and the Coleraine-Portrush branch.

Lough Neagh - the largest lake in the British Isles - laps the southeastern boundary of County Antrim. Its outflow is the Bann, a river that runs north to the Atlantic and supports one of the great salmon fisheries of these islands. The small town of Toome sits at the outflow and is the centre of the fishery. Just below Toome lies Lough Beg, the "Small Lake." Two of Northern Ireland's main ports are in Antrim: Larne, with ferries to Cairnryan in Scotland, and Belfast, which handles about two thirds of Northern Ireland's seaborne trade. Antrim is, in its way, a hinge - between Ireland and Scotland, between Catholic and Protestant histories, between the rural Glens and the urban shipyards of Belfast - and most of its stories are stories of crossings.