How does having spare receptors change ED50 in an irreversible agonist
Irreversible antagonism is like removing receptors.

If there are spare receptors, maximum effect, or any effect level, can be recovered with more ligand (because displacing the irreversible ligand is not necessary, when there are other sites to fill).

$$K_D = \frac{R*L}{RL}$$

A given effect level corresponds to a given $RL$, but with less receptor $R$ available, more ligand $L$ is needed. That is by definition a change in ED50.

If you have no spare receptor (or there is enough antagonist to prevent full effect even with spare receptors), then maximum effect cannot be recovered. There is a new Emax, lower than without antagonist; but without shifting the dose-response curve to the right.

Here is an example in Rang&amp;Dale (8th ed., p. 12, fig 2.6). Curves are labelled with time of incubation with a given quantity of irreversible inhibitor, that is equivalent to varying concentration of irreversible inhibitor at steady state. First two show the situation with spare receptors, and last two show what happens when the original Emax cannot be reached because there are not enough receptors left to elicit maximum response.