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Although she suggested something to the contrary , in recent months Mary Lou McDonald has been consistent that a no-deal Brexit should trigger a border poll.

It's no surprise the taoiseach (Irish prime minister) disagrees.

He argues Northern Ireland cannot be simply grafted on to the Irish Republic in the way that East Germany was assimilated into West Germany.

Instead, Leo Varadkar says an entirely new state with a new constitution would be required, which takes into account the British identity of a sizeable section of its potential future population.

All this would require careful and lengthy preparation.

Ms McDonald turns this argument on its head, insisting that if the Irish government isn't prepared for the prospect of a united Ireland, then it needs to get prepared.

The taoiseach worries such planning could convince unionists that all his careful work on the Brexit border backstop has been part of a land grab rather than - as he insists - a simple defence of the status quo.

Whatever the differences between Fine Gael and Sinn Féin, the debate remains hypothetical until the UK government decides otherwise.

Under the Good Friday Agreement, it's up to the Northern Ireland secretary to decide if there are sufficient grounds to test the will of the people of Northern Ireland on their constitutional status.

The DUP says Prime Minister Boris Johnson told them during recent talks at Stormont that he sees no reason - as things stand - to arrange a border poll.

So both the British and Irish governments are reluctant to see a border poll in the near future.

How sustainable that position will be might depend not just on future opinion polling, but on what happens to the electoral arithmetic in Northern Ireland in any elections after Brexit.