Mr McKay stated the party believed Irish reunification was a matter of “when not if”.
Sinn Fein has been accused by politicians of using Brexit to push for a united Ireland.
The former Secretary of State for Northern Ireland, Lord Hain, suggested the party would benefit from the uncertainty of Brexit.
Speaking on BBC Radio 4’s World at One, Mr McKay, who stood down from Sinn Fein in August last year, said if the party was to gain power it could look to introduce MPs from the north in the Irish parliament, the Dail.
Daithí McKay said Sinn Fein thought Irish reunification was a matter of "when not if"
Certainly from Sinn Fein’s perspective, it is now a matter of when rather than if
He said: “Certainly from Sinn Fein’s perspective, it is now a matter of when rather than if and if Sinn Fein were to go into government in the south, they will have a number of issues on the table in regard to the north that may be the representation of MPs from the north in the Dail itself.
“Sinn Fein are now in the south to stay. It has been part of their strategy for the past ten to 20 years and now they’re finally beginning to see the fruition of that.”
Speaking on Radio 4, he said: “The nationalists and republicans have always wanted a united Ireland and I think that Brexit plays into their hands.
Brexit debate in pictures: Farage blasted by EU boss Wed, April 5, 2017
The debate in the European Parliament in Strasbourg will focus on key issues of the Brexit talks including reciprocal rights for EU citizens, the peace process in Northern Ireland and trade
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Nigel Farage gestures during speeches at the European Parliament in Strasbourg, eastern France
“This border issue is very serious. I do think it plays into Sinn Fein’s hands but I don’t think that is the reason for the impasse that we have seen.
“I think Sinn Fein genuinely want to see a settlement of the restoration of devolved government, as do all the other parties.”
Lord Hain also reiterated how serious it would be if an agreement was not reached, especially due to Brexit.
He added: “The dangers are direct rule, Westminster rule, will be re-imposed, and that would be catastrophic for the continued momentum for the peace process."