Help Macbeth Question?

As it happens, Malcolm brings "Great Birnam Wood to High Dunsinane Hill" and Macduff reveals that he was "not of woman born" but was instead "from ...[his] mother's womb untimely ripped. But how is the manner of these fulfilments ironic, considering the significance Macbeth attached to them when he heard them during his second meeting with the Weird Sisters?