Mide originally referred to the area around the Hill of Uisneach in County Westmeath, where the festival of Beltaine was celebrated.

In the fourth and fifth centuries its territories were taken over by the Uí Néill from Connacht and they pushed out Laigin tribes.

An ogham stone found south of Slane suggests they controlled the Brega area in County Meath together with Carbury Hill and the plains of Kildare.

De Lacy took possession of the kingdom and the dynasty of the Ua Mael Sechlainn or O Melaghlins were forced west and settled on the east bank of the River Shannon in the barony of Clonlonan.

The Diocese of Meath established by the Synod of Ráth Breasail in 1111 had boundaries similar to those of the kingdom.