Ulster, the northernmost of the four historic provinces of Ireland. Ulster was a kingdom of the Gaelic Celts before coming under English control in the 12th century. It resisted English rule until the 17th century, when large numbers of Scottish Presbyterians settled there. From then on, as a predominantly Protestant province, it sided with the British government against Irish independence. Six of Ulster's nine counties make up Northern Ireland, which is sometimes referred to as Ulster; the other counties are part of the Republic of Ireland.