Former county, western central Northern Ireland. In the 1973 administrative reorganization of Northern Ireland, the county was divided into several smaller districts. The O'Nialls (or O'Neills) ruled the territory from the 5th to the 16th century AD. Subsequently the vast estates passed to the English crown and were divided and granted by the king under the scheme for the Plantation of Ulster. Royalist forces established fortifications, and the area was colonized.