She's the first woman in modern history to lead the nationalist party, as Gerry Adams steps down after 34 years.
Mary Lou McDonald was formally elected as the new leader of Sinn Féin on Saturday, taking over from Gerry Adams who led the nationalist party for over three decades.
"As a woman now charged with leading Irish republicanism forward, I am very conscious that I stand on the shoulder of giants," McDonald told party members in a speech in Dublin.
The handover could help Sinn Fein's ambition of ruling on both sides of the Irish border.
Adams had led the party since 1983. He announced his intention to step down in November.
The election of a woman from a younger generation, who had no direct involvement in the 30-year Northern Irish conflict, represents a major break with the past for the former political wing of the Irish Republican Army (IRA).
Unlike Adams and former North Ireland deputy first minister Martin McGuiness, who died last year, McDonald had no personal ties with the IRA or the bloody campaign it waged against British rule.
She was born in Dublin and joined Sinn Feinn after the 1998 Good Friday peace agreement, having previously belonged to Fianna Fail, a conservative establishment party.
The party's last female president was Margaret Buckley, who led the party between 1937 and 1950.
The change of leadership is now seen as an opportunity for the left-wing party to reposition itself closer to the political mainstream and win more supporters across the island it ultimately wants to unite.
"The war is over," McDonald said in her speech. "Now as a new generation takes the reins of leadership, our job is to bring innovative and modern ways of advancing our politics. Now is the time for fresh thinking and bold ideas."
Sinn Fein has shared power in the British province of Northern Ireland since 2007 and is in talks to restore devolved government there, but it has never governed in the Republic, where it's the third largest party.