On Wednesday at a news conference Mary Lou McDonald turned the screws on Simon Coveney over the Katherine Zappone debacle. The intervention brought an Irish Independent headline ‘Sinn Féin leader Mary Lou McDonald calls on The Taoiseach to sack Foreign Affairs Minister.” Her beleaguered target had already moved on from eating humble pie to eating mud.   The instant effect of the Sinn Féin leader’s action was it convinced the three coalition parties there could be no execution.   The McDonald intervention amounted to a demand that wouldn’t be delivered. Maybe one reason for it was it plays well with most sections of a support…