PSNI Deputy Chief Constable Drew Harris has been appointed the new Garda Commissioner.
The vacancy was created last September following the resignation of Noirin O'Sullivan.
She had faced calls to stand down from the top policing post after her handling of police whistle-blowers.
It is understood the Garda was keen to appoint someone from outside the organisation as the force needs widespread reform in order to regain public trust.
Mr Harris, a police officer since 1983, was appointed as deputy chief constable of the PSNI in October 2014.
He will become the first "outsider" to take up the role.
The salary of the Garda Commissioner was increased to €250,000 a year from €180,000 in order to attract interest in the position.
Mr Harris' father, RUC Superintendent Alwyn Harris, was killed by the IRA in 1989. A Semtex bomb exploded under his car as he was on his way to church with his wife. She survived the attack.
Congratulations to Drew Harris on his appointment as commissioner of An Garda SÃochÃ¡na. Look forward to continuing a close working relationship with him. https://t.co/SVAcgJXizT pic.twitter.com/qKKQTdsWVE— Sir George Hamilton (@SirGHamilton) June 26, 2018
Chief Constable George Hamilton said: “I am delighted that Drew Harris will be the next Garda Commissioner.
“While there is no doubt that the Police Service of Northern Ireland will be a poorer place without his contribution; he will remain a colleague when he takes over the leadership of An Garda Síochána.
“This appointment can only serve to build on what is already a close working relationship between the PSNI and An Garda Síochána. The mutual desire of both organisations to keep people safe will support us in overcoming the many challenges we jointly face, including Brexit.
“In his 34 years as a police officer in both the RUCGC and the PSNI, Drew Harris has been driven by a commitment to protect the most vulnerable in our community. I have no doubt that this will continue to be his ambition as Garda Commissioner.
"He is a man of unfathomable strength, humility and grace and I have been privileged to have him serve as my Deputy Chief Constable for almost four years.
“I wish him every success and look forward to working with him as Commissioner.”
Secretary of State for Northern Ireland Karen Bradley congratulated Mr Harris on his appointment.
"This is a critically important post and I am delighted that An Garda Síochána will be able to draw on Drew's vast policing experience which is as broad as it is deep.
"I would also like to place on record the UK Government's gratitude for his personal and professional contribution to public service and policing, most recently as Deputy Chief Constable of the PSNI," she said. "We look forward to continuing to work closely with him in this new role."
Commenting on the appointment today of Drew Harris to the post of Commissioner of An Garda SÃochana, the Secretary of State for Northern Ireland, Karen Bradley MP, said: pic.twitter.com/GzdkSglMT4— Northern Ireland Office (@NIOgov) June 26, 2018
Irish Justice Minister Charlie Flanagan said: "Drew takes up office at a time of major reform and investment which will redefine An Garda Síochána as an organisation.
"As we approach the centenary of the establishment of An Garda Síochána, the organisation is on the cusp of significant change.
“I am confident that Drew Harris has the necessary ability and vital policing, security and change management experience required.
"He will bring these talents to his leadership of An Garda Síochána - one of our most important institutions of state - along with a commitment to serving the public in the best tradition of the Gardaí."
An expert Commission on the Future of Policing is in the final stages of its work and its report will chart a new model for Irish policing.