Martin McGuinness looks to have finished third in the Irish presidential elections behind Michael D Higgins and Sean Gallagher.
Mr McGuinness, who stepped down as Northern Ireland's Deputy First Minister to run, secured a huge boost for his party by topping the poll in the Donegal North-East constituency.
Gerry Adams, his party's president, said the support would bring politics in Northern Ireland and the Republic closer.
"I think what we have done is narrow the gap between politics in the north and the south," Mr Adams said.
Mr McGuinness phoned Mr Higgins to offer congratulations.
"He will make a fine president and I wish him well for his seven years in the Aras," the Sinn Fein candidate said.
"I am delighted with the strong vote I have received. My message of positive leadership, patriotism and commitment clearly was resonating with tens of thousands of ordinary Irish people. I believe that Irish people do want a new type of politics and a new Republic based upon equality and respect."
Sinn Fein vice president Mary Lou McDonald said Mr McGuinness's campaign had succeeded in raising issues that were important to Sinn Fein and broke new ground for the party.
"For Sinn Fein this is a milestone election," she said. "There was a time - and it's not so long ago - when republicans would have been considered almost a marginal voice in southern politics and today we changed that."
Ms McDonald denied that the furore over Mr McGuinness's IRA past would damage the party in the Republic, noting that his chief critic, Fine Gael's Gay Mitchell, had polled poorly.