Why did Northern Ireland include six of Ulster’s nine counties?

The inclusion of six of Ulster’s nine counties in Northern Ireland was primarily driven by the desire of the British government and Ulster unionists to establish a territory where a Protestant unionist majority could be guaranteed.12

Under the Government of Ireland Act of 1920, the British Parliament divided Ireland into two self-governing regions. While the province of Ulster historically consisted of nine counties, the six selected—Antrim, Down, Armagh, Londonderry, Tyrone, and Fermanagh—represented the largest area that could be carved out while maintaining a secure, permanent Protestant majority.12

The remaining three counties of Ulster—Donegal, Cavan, and Monaghan—had significant Catholic nationalist majorities and were consequently excluded from the new state, becoming part of what would eventually be the Republic of Ireland.12