The 38-year-old is the Republic’s youngest leader. Mrs Long said she hoped he would be able to maintain the positive relations between north and south, particularly in the light of future challenges.
“I would like to congratulate Leo Varadkar on his appointment and I look forward to working with him and his team,” she said.
“Northern Ireland and the Republic have a good relationship and I am confident will continue under his stewardship. That is particularly important given the challenges we face in the near future – especially in relation to Brexit and the future of the powersharing institutions.
“That requires proactive cooperation, and I look forward to working with the new Taoiseach as with his predecessors to meet those challenges.”