According to the BBC, Catherine Connolly was sworn in as the 10th President of the Republic of Ireland in Dublin Castle.
An independent candidate, Connolly was elected in late October, defeating Fine Gael candidate Heather Humphreys.
Mike Higgins, whose 14-year term expired at midnight, succeeded Connolly, 68, as President.
The President of the Republic of Ireland is the head of state and commander-in-chief of the armed forces, but the role of the President is largely ceremonial.
Connolly has been a member of the Irish Parliament since 2016.