Six of Ulster's nine counties, Antrim, Armagh, Derry (formerly known as County Coleraine before being renamed during the Plantation of Ulster), Down, Fermanagh, and Tyrone (Tír Eoghain), form Northern Ireland, and remained part of the United Kingdom after the partition of Ireland in 1921. Three Ulster counties, Cavan, Donegal, and Monaghan form part of the Republic of Ireland. About half of Ulster's population lives in Antrim and Down. Many inhabitants (especially unionists) refer to the six-county Northern Ireland as "Ulster."

Most people in Ulster speak Mid Ulster English, Ulster Scots, Hiberno English or Irish as their primary language. Irish probably comes second by number of fluent speakers. Cantonese forms the third most common language, mostly due to the considerable Chinese community of Belfast, the province's largest city. Belfast has more Chinese restaurants per capita than any other European city.

Some sources refer to the inhabitants of Ulster as Ultonians - from the traditional Latin form of the name of the province: Ultonia.

The biggest lake in Ireland, Lough Neagh, lies in eastern Ulster. The province's highest point, Slieve Donard (848 metres), stands in Down. The most northerly point of Ireland, Malin Head, and the highest (601 metres) sea cliffs in Europe, at Slieve League, both form part of Donegal. The longest river in Ireland, the Shannon, rises in Cavan. Volcanic activity in eastern Ulster led to the formation of the Antrim Plateau and the Giant's Causeway, one of Ireland's three UNESCO World Heritage Sites. The geographical centre of Ulster lies between the villages of Pomeroy and Carrickmore in Tyrone.

The province's early story extends further back than written records and survives mainly in legends such as the Ulster Cycle. In early medieval Ireland, the Uí Néill (O'Neill) dynasty dominated Ulster from their base in Tír Eóghain (Eoghan's Country) - modern Tyrone. After the Norman invasion of Ireland in the twelfth century, the east of the province fell by conquest to Norman barons, first De Courcy (died 1219), then Hugh de Lacy (1176-1243), who founded the Earldom of Ulster - based around the modern counties of Antrim and Down. However, by the end of the 15th century the Earldom had collapsed and Ulster had become the only Irish province completely outside of English control.

In the 1600s Ulster was the last redoubt of the traditional Gaelic way of life, and following the defeat of the Irish forces in the Nine Years War (1594-1603) at the battle of Kinsale (1601), Elizabeth I's English forces succeeded in subjugating Ulster and all of Ireland. The Gaelic leaders of Ulster, the O'Neills and O'Donnells, finding their power under English suzerainty limited, decamped en masse in 1607 (the Flight of the Earls) to Roman Catholic Europe. This allowed the Crown to settle Ulster with more loyal English and Scottish planters, a process which began in earnest in 1610.