thermodynamics - Why can one use this equation for entropy if this process is irreversible?

The equation

$$dS = \dfrac{dQ}{T}$$

is said do hold only on reversible processes. Indeed this is almost always emphasized by writing

$$dS = \dfrac{dQ_{\mathrm{rev}}}{T},$$

to be clear that this is during one reversible process.

Now there are some irreversible processes on which this is used. For instance, if one mass $m$ of a substance melts at temperature $T_0$ and if it has latent heat of fusion $q_L$ then it is usually computed that

$$\Delta S = \dfrac{mq_L}{T_0}.$$

Another example is when heat enters a system at constant temperature. In that case if the heat is $Q$ we have

$$\Delta S = \dfrac{Q}{T}.$$

All these processes are clearly irreversible. It is intuitively clear, but more than that we have $\Delta S > 0$ in all of them.

Still we are finding $\Delta S$ using

$$\Delta S = \int \dfrac{dQ}{T},$$

and it is obviously that this integral is being carried along irreversible processes in the examples I gave.

In that case, whey can we use this formula to find the change in entropy if the processes are irreversible?