Leo Varadkar has become the youngest ever Taoiseach, after being given his Seal of Office by President Michael D Higgins earlier.
The former Social Protection Minister also won the Dáil vote to be elected Taoiseach by 57-50 votes - with 45 abstentions.
He is also the first gay man to hold the office. His family and partner Matt Barrett were in Áras an Uachtaráin to watch him take office.
Leo Varadkar's family and friends, including parents Ashok and Miriam with his partner Matthew Barrett, at Aras an Uachtarain in Dublin to see him receive his Seal of Office as Taoiseach from President Michael D Higgins | Image: Brian Lawless/PA Wire/PA Images
World leaders have been reacting with congratulations, while the global media is focusing on his sexual orientation.
The American Chamber of Commerce Ireland has welcomed his election, with its President James O’Connor saying: "There is no doubt that the uncertainty of Brexit and a new US administration means that Ireland has a more important role than ever to play in the US-EU relationship.
"As the leadership voice of over 700 US companies with EMEA and International Headquartered Operations in Ireland, the American Chamber of Commerce looks forward to constructively engaging with Mr Varadkar and his Government on the issues of concern to the US FDI sector in Ireland".
Leo Varadkar is congratulated as he leaves Leinster House in Dublin after he has been elected as Taoiseach | Image: Rollingnews.ie
President of the European Council Donald Tusk, tweeting in Irish, also sent his congratulations to Mr Varadkar.
He says: "Ireland is rightly admired for its strong economy; vibrant and open society; and stalwart commitment to Europe.
"I want to assure you of the great sensitivity amongst your partners in the EU27 to the challenge now facing the island of Ireland after the withdrawal of the United Kingdom from the European Union in 2019.
"Addressing these unique circumstances is a priority for the first phase of talks. Flexible and imaginative solutions will be needed and I am fully committed to working closely with you in this respect.
"I look forward to welcoming you to the European Council next week where we will discuss security and defence; jobs, growth and competitiveness; migration; and external relations."
Comhghairdeas leis An Taoiseach Leo Varadkar @campaignforleo. Is mise le meashttps://t.co/Yo4cleTi7C pic.twitter.com/I0omsStAgn
— Donald Tusk (@eucopresident) June 14, 2017
Several global news agencies are also covering the story.
AFP runs with the headline: Ireland's first gay prime minister enters office.
It says: "Although regarded as relatively liberal on social issues such as gender equality and abortion rights, Varadkar has been criticised by opposition parties for his right-wing economic views."
Reuters also focuses on Mr Varadkar's sexuality, saying: Ireland elects first gay prime minister Leo Varadkar.
The article says: "Despite inheriting Europe's fastest-growing economy, he will face immediate challenges in the shape of neighboring Britain's exit from the European Union, a political crisis in Northern Ireland and a housing crisis at home."
But in a second piece by Reuters - featured in The New York Times - it looks at his Indian roots, featuring the headline: Immigrant's Son Becomes Ireland's Youngest PM.
Britain's The Guardian says one of the first world leaders Mr Varadkar plans to call is Theresa May, while also focusing on the issue of the 8th amendment.
The Telegraph headline reads: Ireland elects first gay prime minister Leo Varadkar - with the piece talking about his career, personal life and questioning his longevity in the job.
While the UK's Gay Times magazine is also running the story, saying: Ireland now officially has its first ever openly gay prime minister.
It notes that Mr Varadkar joins Luxembourg’s Prime Minister Xavier Bettel as the only other openly gay world leader currently serving.
