province in Ireland

Ulster is one of the four historical provinces which make up the island of Ireland. The others are Leinster, Munster and Connacht.

Ulaidh  (Irish)
Ulstèr (Ulster-Scots)
Location of Ulster

There are nine counties in Ulster. Three of them are in the Republic of Ireland:

The other six counties make up all of Northern Ireland, which is part of the United Kingdom:

Due to the fact that most of Ulster is in Northern Ireland, the name "Ulster" is also sometimes used to mean Northern Ireland, even though Northern Ireland is only part of Ulster. This is called totum pro parte in Latin which means "the whole for a part".