Michael D Higgins will be Ireland's ninth president after one of the most remarkable political comebacks in the history of the state.
The Labour Party veteran, poet and campaigner, seized an unprecedented swing in support following the spectacular implosion of his biggest rival on live television.
Mr Higgins declared his triumph was built on a left-leaning campaign while his seven-year term will be marked by inclusion, ideas and transformation.
The 70-year-old's resounding victory was obvious within an hour of ballot boxes being opened in a massive voter swing just days after favourite Sean Gallagher publicly derailed during a TV debate.
"I feel a little overwhelmed," Mr Higgins said.
"I'm very, very happy. It is something I prepared for, something I thought about for a long while.
"I am very glad as well that it is a presidency built on a campaign that emphasised ideas. I hope it will be a presidency that will enable everybody to be part of and proud of."
Mr Higgins secured the victory for Labour - the second in a day as the party edged towards a by-election win - after the other six candidates conceded defeat.
Mr Gallagher, the opinion poll topper with a 15-point margin as recent as Sunday, saw his support vanish to 28% in a stunning defeat blamed on his ties to Fianna Fail, the party most associated with Ireland's economic demise.
Mr Higgins will be inaugurated on Armistice Day, November 11, the day after current president Mary McAleese leaves office.
Sinn Fein's Martin McGuinness, who was ran third in the poll securing a tactical victory, dropped the bombshell that Mr Gallagher had requested and personally received a €5,000 cheque from a businessman with a conviction for tax fraud and fuel smuggling.
With his wife Trish by his side, Mr Gallagher admitted it had been an enduring campaign before he paid tribute to Mr Higgins.
Mr McGuinness, who is to return to his role as Northern Ireland's Deputy First Minister, later dismissed suggestions that he was in the race for tactical, party political reasons.
"I was in the contest to win," he said.
Two independents at the bottom of the polls, Mary Davis, who headed the Irish division of Special Olympics, and Dana Rosemary Scallon, former Eurovision winner and Eurosceptic MEP, were excluded on the first count.
Labour, the junior party in coalition with Fine Gael, celebrated a double whammy at the polls with the certain prospect that councillor Patrick Nulty will win the seat in Dublin West left empty following the death of former finance minister Brian Lenihan last year.
It will be the first time since 1982 a governing party has won a by-election.
But Gay Mitchell was on course for the worst performance in history by a candidate from Fine Gael.
His failure to register left him vying with Senator David Norris, a former Trinity professor and Joycean scholar, for the fourth and fifth spots. The Senator was first candidate to declare Mr Higgins' win at in the morning.
Justice Minister Alan Shatter was again forced to deny allegations that Fine Gael failed to offer its full support to presidential flop Mr Mitchell.
He said he did not think Mr Mitchell's defeat had anything to do with the party, saying the presidential election is more about personality than politics.