The headwaters of the Irwin are located below Canna near Pindar. The river flows west until discharging into Arurine Bay near Dongara.
The river has four tributaries: Lockier River, Sand Plain Creek, Nangetty Creek and Mullewa Creek.
The river occasionally floods as it did in 1945 following a severe storm that swept over the area. The river broke its banks and caused extensive damage including the loss of 450 sheep that were swept away from a farm that straddled the river.