When Ireland divided into two countries which part stayed with Great Britain?

Historically, The Reublic of Ireland was divided in to four provinces, viz., Ulster(North), Munster(South), Leinster (East), and Connacht/Connaught (West).

Each province was divided into counties.

The province of Ulster consisted of nine counties.

On division in 1922, the international border between the UK and the Irish free State separated the nine counties of Ulster, into six counties of Northern Ireland(UK) and three counties into the Irish Free State..

The six counties of Northern Ireland (UK) are Antrim, Down, Londonderry, Tyrone, Armagh and Fermanagh.

The three counties of Ulster that remain in the Republic of Ireland are Monaghan, Donegal, & Leitrim.