Here is a more conceptual approach to Clifford theory. Let me work with a slightly more general setup: namely, suppose we have a short exact sequence
$$1 \to N \to G \to H \to 1$$
of finite groups, which does not necessarily split. What does the representation theory of $G$ look like, in terms of $N$ and $H$? This question can be answered categorically as follows. The idea is to think of $G$ as the homotopy quotient $N//H$ of $N$ with respect to a (categorical) action of $H$. This gives an equivalence
$$\text{Rep}(G) \cong \text{Rep}(N)^H$$
where $(-)^H$ denotes taking homotopy fixed points. As a category,
$$\text{Rep}(N) \cong \prod_{\hat{N}} \text{Vect}$$
where $\hat{N}$ denotes the set of isomorphism classes of irreducible representations of $N$. The action of $H$ on this category breaks up based on the orbits of the action of $H$ on $\hat{N}$, so from now on we restrict our attention to a single orbit. If $\chi$ is an irrep in this orbit then we denote this orbit by $H\chi$.
Let $H_{\chi}$ denote the stabilizer of $H$ acting on $\chi \in \hat{N}$. Then we have an equivalence
$$\left( \prod_{H \chi} \text{Vect} \right)^H \cong \text{Vect}^{H_{\chi}}$$
where this copy of $\text{Vect}$ is spanned by $\chi$. So now it suffices to understand the homotopy fixed points of $H_{\chi}$ acting on a single copy of $\text{Vect}$.
Actions of $H_{\chi}$ on $\text{Vect}$ are classified by classes in $H^2(H_{\chi}, \mathbb{C}^{\times})$. Given a $2$-cocycle $c$ representing such a class, the category of homotopy fixed points is the category of projective representations of $H_{\chi}$ with $2$-cocycle $c$. I don't know if there's an easy recipe for computing these $2$-cocycles; Geoff Robinson also indicates that this is the hard step.
Anyway, once the irreducible projective representations of $H_{\chi}$ with $2$-cocycle $c$ have been determined, passing through the equivalences we've been using reproduces Geoff Robinson's answer (with weaker hypotheses): irreps of $G$ are classified by projective irreps of the $H_{\chi}$ with appropriate cocycles. From here the nicest thing that can happen is that
$$H^2(H_{\chi}, \mathbb{C}^{\times}) = 0$$
for all $\chi$, which in fact occurs in this example: $H_{\chi}$ is either trivial or cyclic of order $2$. So as Geoff Robinson and Derek Holt indicate, in this case it suffices to understand the orbits of the action of $\mathbb{Z}_2$ on the set of irreps of $M_{12}$.
For some blog references see here, here, here, here, and here, but the sequence isn't done yet. For references in the literature I think Ganter and Kapranov is relevant but I haven't looked at it in detail.