In non-competitive antagonism a ligand (agonist) and an irreversible antagonist compete for the binding site on the receptors. However, once the irreversible antagonist has bound, the receptor remains occupied and can no longer respond to the binding of the agonist.
The graph shows the specific binding of the ligand [agonist] alone, and in presence of increasing concentrations of irreversible antagonist. In the absence of antagonist, the ligand is able to bind all the receptors that are present (black line).
Thus, non-competitive antagonists reduce agonist maximal response (efficacy), but not its affinity.