Sonnagh fort is a ringfort (rath) and National Monument located in County Longford, Ireland.

It stands in a commanding place overlooking the eastern plains extending to County Westmeath.

The ringfort is a large bivallate enclosure with double bank and ditch.

[4] While house type varied, most were made of wood and were usually of post and wattle construction.

The walls of the houses consisted of a double row of wattle spaced about 20 cm apart with a cavity filled with straw and bracken for insulation.