Labour Party leader Brendan Howlin has called for an explanation as to why a visit by the Taoiseach to the Hungarian leader Victor Orban was not revealed until now.
Mr Howlin accused Leo Varadkar of not telling the Dáil on December 13th of his intentions to visit Mr Orban.
In response to a parliamentary question from TD Richard Boyd-Barrett last month, Mr Varadkar made no mention of his intention to visit the Hungarian prime minister in January.
A spokeswoman for the Taoiseach responded by saying the visit was only organised just before Christmas.
Mr Orban has been at loggerheads with the European Union (EU) for a long time over his anti-migrant stance and for measures his critics say are undemocratic.
Mr Varadkar will travel to Budapest on Thursday to meet with Mr Orban. The Taoiseach said his visit was part of ongoing discussions with European leaders as Brexit negotiations enter the next phase.
He confirmed that he will raise issues with Mr Orban with which he does not agree. “The Taoiseach strongly believes in a policy of engagement when it comes to foreign and European affairs,” the spokeswoman explained.
“That inevitably involves meeting people whose policies you don’t agree with and values you may not share and of course a willingness to raise those issues in meetings which the Taoiseach will do.”
Mr Howlin said Mr Varadkar was now one of the few EU leaders to directly visit Mr Orban.
He called on Mr Varadkar to denounce the policies pursued by the Hungarian government.
Otherwise, he suggested the visit will be viewed by Mr Orban as an “implicit endorsement” of his policies which Mr Howlin described as “anti-democratic, against EU values and the rule of law”.
Mr Howlin added: “Mr Orban has removed constitutional checks and balances alongside taking control of state media and the judiciary, and has been seeking to take control of an independent university.
"He has stoked right wing populism, and anti-migrant sentiment with his publicly funded 'Let's Stop Brussels! ' campaign that saw a questionnaire sent to every household asking how Hungary should deal with immigration and other EU related issues he disagreed with.
"I hope the Taoiseach will have the courage to defend both the values Ireland and the EU have upheld when he meets with Mr Orban tomorrow and to criticise the divisive path that Hungary is pursing within the EU."
Mr Varadkar will then travel on to Bulgaria to meet the Bulgarian prime minister Boyko Borissov. Bulgaria has assumed the EU Presidency for the next six months.
The two men will discuss the reform of the EU’s financial framework post 2020 and the future of the Common Agricultural Policy.
Mr Varadkar described both visits as a “good opportunity to develop a deeper understanding of the challenges and opportunities facing our countries and the EU in 2018.”