The Irwin River is about 350 km north of Perth in the Mid West region of the state. The catchment borders the Arrowsmith River to the south, and the Greenough River to the north. The average annual rainfall varies between about 450 mm on the coast, falling to about 400 mm mid catchment (Mingenew), and reducing even further in the east of the catchment to about 330 mm.
The headwaters of the Irwin River are located about 30 km east of Mullewa. From there the river flows 140 km south west, discharging to the Indian Ocean just south of Dongara.
The catchment has a land area of almost 6,100 km2 which is mostly used for unirrigated cropping with some grazing, and grazing land uses are mostly located in the western (lower) half of the catchment. Gaining sections of the river, where ground water supplements surface water flows, are found east of Mingenew on the Irwin River and a major tributary, the Lockier River, as well as around the Irwin townsite, which is located in the lower reaches of the catchment about 15 km inland of Dongara.