Former Sinn Fein leader Gerry Adams.

Writing on his Léargas (Insight) blog, Mr Adams said it is not only civic nationalism that is calling for a border poll.

"Increasingly there are also voices being raised from within unionism on the issue of unity. In part this is because of the shambles that is Brexit and the social, political and economic implications this will have for all of society in the North and across our little island," the Louth TD said.

But Mr Adams warned that it would be "stupid" to hold a referendum on Irish unity before plans are in place regarding how the transition to a united Ireland would work in practical terms.

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Stressing that "rhetoric is not enough", Mr Adams said the Irish government has "a duty to plan for unity" by establishing a 'Forum for Unity' to "open up consultations" and a "process of dialogue" around the subject.

"This needs planned now. Not after the referendum," he said. "That is the one big lesson of Brexit. A referendum without a plan is stupid. So a referendum on unity must be set in a thoughtful inclusive process which sets out a programme of sustainable options. Including phases of transition."

The former West Belfast MP and MLA said there would have to be "a transition phase" after any referendum in favour of Irish unity, and "protections" put in place to "obviate any fears" of unionists.

"No one I know who wants a United Ireland believes that it should be any other than a warm house for unionism, built on a foundation of equality and inclusiveness. This is evident in the debate taking place on the unity issue," he said.

"What accommodations are needed to persuade political unionism that a United Ireland can work for it? Key to this is the need for it to be an agreed shared Ireland. What happens to the political institutions established by the Good Friday Agreement?

"Winning support for a United Ireland is not just about persuading unionists although that is crucial. Everyone needs to be convinced of the advantages of unity – personal, economic, wages, health provision, environmental, cultural, peace, prosperity.

"There will be a referendum on Irish unity. I am confident of this. Winning that referendum is the biggest single challenge facing United Irelanders."

Meanwhile, Meath West TD and leader of Aontú, Peadar Tóibín, has called for a "New Irish Forum on Irish Unity" and "practical planning to begin now to map out the process of economic and service convergence north and south."

Stressing that a number of high profile unionists have stressed the need for unionism to prepare itself for a border poll, he said: "It’s no doubt that the demographic changes and Brexit has driven some of this increased support for Irish unity. Unity is the key to unlocking the potentially catastrophic Brexit conundrum. It will also help resolve the ongoing dysfunction and stalemate in Stormont. It would mean a larger domestic enterprise market, larger representation in the EU and reduced cost of delivering public services and infrastructure.

“There is no doubt that there would be significant challenges created by a transition towards unity. However, the government’s policy of ignoring public opinion, ignoring the significant political change occurring in the North and ignoring the opportunity to remedy the damage of Brexit is a serious mistake."