Ion acoustic solitary waves with arbitrary amplitude have been investigated using the Sagdeev potential (SP) method in a collisionless plasma consisting of non-isothermal electrons, positrons, and positive ions. The SP has been obtained and the dependence of large amplitude solitary structures on various plasma parameters such as ion drift velocity ( v 0 ) $({v}_{0})$, non-thermal parameter ( beta ) $(\beta )$, electron to positron temperature ratio ( sigma ) $(\sigma )$, positron density ( p ) $(p)$, and Mach number ( M ) $(M)$ have been studied. It is shown that, for the same values of plasma parameters, accounting for ion drift velocity greatly corrects the solitary wave profile.