  In this letter, we consider a multicast system where a single-antenna
transmitter sends a common message to multiple single-antenna users, aided by
an intelligent reflecting surface (IRS) equipped with $N$ passive reflecting
elements. Prior works on IRS have mostly assumed the availability of channel
state information (CSI) for designing its passive beamforming. However, the
acquisition of CSI requires substantial training overhead that increases with
$N$. In contrast, we propose in this letter a novel \emph{random passive
beamforming} scheme, where the IRS performs independent random reflection for
$Q\geq 1$ times in each channel coherence interval without the need of CSI
acquisition. For the proposed scheme, we first derive a closed-form
approximation of the outage probability, based on which the optimal $Q$ with
best outage performance can be efficiently obtained. Then, for the purpose of
comparison, we derive a lower bound of the outage probability with traditional
CSI-based passive beamforming. Numerical results show that a small $Q$ is
preferred in the high-outage regime (or with high rate target) and the optimal
$Q$ becomes larger as the outage probability decreases (or as the rate target
decreases). Moreover, the proposed scheme significantly outperforms the
CSI-based passive beamforming scheme with training overhead taken into
consideration when $N$ and/or the number of users are large, thus offering a
promising CSI-free alternative to existing CSI-based schemes.
