Drew Harris began his new career as Garda Commissioner at one minute past midnight.
Acting commissioner Donall O Cualain stepped down from his role at 00.01am, and in line with the Garda Siochana Act, Mr Harris immediately took up the role.
The attestation took place in Kevin Street Divisional Headquarters.
In keeping with tradition for such occasions, it was a private meeting with a small number of people present.
Mr Harris (53) was attested by a Peace Commissioner and signed the Garda Code of Ethics and the Official Secrets Act.
He will now take charge of the general direction and control of An Garda Siochana from his office in Garda Headquarters in Phoenix Park.
Mr Harris, the former deputy chief constable of the Police Service of Northern Ireland (PSNI), is a father-of-four who joined the Royal Ulster Constabulary (RUC) in 1983.
His father, RUC superintendent Alwyn Harris, was murdered in an IRA car bombing in 1989.
Mr Harris has been PSNI deputy chief constable for the past four years. He has relinquished his sworn oath to serve the UK and has applied for an Irish passport.
With a salary of €250,000, he will travel in an armoured vehicle, under escort, as he is considered a target for dissident republicans.
The appointment of Mr Harris has been the subject of much controversy.
Last month, Ciaran MacAirt, whose grandmother was killed by a loyalist bomb, applied to the High Court to conduct a judicial review into the appointment of Mr Harris.
He argued that Mr Harris' oath to the UK's Official Secrets Act meant he could not independently stand over an inquiry involving alleged collusion between British security services and loyalist terrorists, but the court rejected this.