According to historian C. Thomas Cairney, the MacNamaras were one of the chiefly families of the Dal gCais or Dalcassians who were a tribe of the Erainn who were the second wave of Celts to settle in Ireland between about 500 and 100 BC.

They are related to the O'Gradys, also descended from the Uí Caisin line of the Dál gCais.

The name began with the chieftain Cumara, of Maghadhair in county Clare.

Cumara is a contracted form of Conmara – hound of the sea.

[2] The name is a contraction of "Mac Cú Na Mara" meaning "Son of the Hound of the Sea".