When Macduff is knocking at the gate he tells the Porter, " He (Duncan) did command me to call timely on him: I have almost slipped the hour."

Call timely means that Duncan had asked Macduff to wake him early.

How is this ironic? Hint: Think about how/why Macduff was late and what Macbeth and Lady Macbeth were doing right after the murder and they hear the knocking.