To improve spectrum efficiency， this paper proposes a robust multicast transmission algorithm for intelligent reflecting surface （IRS） aided cognitive satellite and terrestrial network （CSTN）. Specifically， the satellite uses multicast technology to serve multiple primary users， while the terrestrial base station （BS）， sharing spectrum resources with the satellite network， serves direct users and blocked users through space division multiple access technique and intelligent reflecting surfaces， respectively. Then， a joint optimization problem is formulated to minimize the BS transmit power， while satisfying the outage constraints of both the signal-to-interference-plus-noise ratio of terrestrial users and the interference power of the primary users. To address this nonconvex problem， the nonconvex outage constraint is first transformed into a deterministic form with the assistance of the cumulative distribution function of the exponential distribution. Then， a robust beamforming algorithm combining alternating optimization with semi-positive definite relaxation is proposed to obtain a solution with better performance. Computer simulation results demonstrate the robustness and superiority of the proposed algorithm.