In the original Gaelic divisions Mide was in fact a fifth province and incorporated what is now Westmeath, as well as the present Co. Meath, and large parts of Cavan and Longford. Westmeath covers what was the northern part of the province, known as Teffia. Granted to the de Lacys in the twelfth century, it was never defended or exploited as intensively as the de Lacy possessions further to the east, and by the end of the thirteenth century it had reverted to the Gaelic Irish. Chief among these were the McGeoghegans, the O'Currys and the O'Mulleadys. It was separated from Meath and became a county in its own right in 1541. After the final defeat by Cromwell, there were huge confiscations of land in the region and virtually none of the original proprietors survived.
In addition to the usual destinations of Irish emigrants - England, the U.S., Canada and Australia - many emigrants from Westmeath made their way to Argentina in the nineteenth century, where their sheep-rearing skills were much valued.
Surnames associated with the county include Cumisky, Dalton, Farrell, Grehan and Scally.