Irish presidential elections: Who are the six candidates?
The Derry businessman (61) is best known as an investor on RTÉ's Dragons Den. He lived in Sydney for more than a decade from 1981 before later moving to Atlanta in the US where he founded a global recruitment company.
He has backed voting rights for Irish emigrants and insisted he will not take a salary if he wins.
However, he has drawn criticism for claiming that the Travelling community is not an ethnic group but "basically people camping in someone else's land".
The 58-year-old entrepreneur from Co Kildare is an investor on Dragon's Den.
A hunting enthusiast, Mr Duffy has suggested he could spearhead the Republic's Brexit negotiations - despite the fact that the position of president is essentially a ceremonial one.
"I feel I have the particular skill set that would help in these situations," he told his campaign launch.
A founder of suicide prevention charity Pieta House, the 60-year-old has served as a senator since May 2016.
She has indicated she voted No in the Republic's abortion referendum but would be happy to sign abortion legislation into law as president.
Ms Freeman was criticised after it emerged her friend Des Walsh, an Irish businessman living in Los Angeles, loaned her €120,000 to finance her campaign.
Mr Walsh was president of Herbalife, a "global nutrition company" which was criticised by US government consumer protection body the Federal Trade Commission.
The Co Cavan businessman (56) was tipped to be president in 2011 but lost out to Mr Higgins.
A Fianna Fáil member for 30 years, he has stood as an independent candidate in both elections.
Mr Gallagher came to public prominence as an investor on RTÉ's Dragons Den - one of three candidates linked to the programme.
If elected, he has promised to campaign for a united Ireland. He also described Brexit as a mistake and said the Republic needs to strengthen its ties with the UK.
Michael D Higgins
The president is the first incumbent to face an election for a second term since 1966. A popular president, the Co Clare-raised 77-year-old, looks almost certain to be re-elected.
In 2014 he made history by becoming the first Irish president to make a state visit to the UK.
The only strong criticism he has faced during the campaign is over complaints about his use of public funds and the government jet.
He claimed he had to travel to Belfast by private jet following advice from his office on security issues.
Questions were also raised after it emerged the president receives an unaudited annual allowance of €317,000 a year.
The allowance was also paid to his predecessor Mary McAleese from 1998.
Liadh Ní Riada
The Sinn Féin candidate is an Irish language campaigner who sat on the board which set up TG4.
An MEP for Ireland South since 2014, the 51-year-old was previously the party's Irish language officer.
She has strongly criticised President Higgins' use of public money, including his use of the government jet to fly to events across the Republic at the taxpayers' expense.