Presidential candidate Liadh Ní Riada has called on Michael D Higgins to respond to questions about his use of public money, in a letter to the incumbent this morning.

Mr Higgins was criticised over the weekend for his comments about the use of a jet to Belfast, which he said was employed for security reasons.

But the PSNI has disputed the claim that they could not provide the security for him to travel by road.

Ms Ní Riada, Sinn Féin's presidential candidate, said she sent the letter to the sitting President "in relation to a number of very serious questions" that were raised over the weekend.

"Among the issues which have been raised in the media were the use by the President of the Government's aircraft at huge public expense to fly within the island of Ireland, to places like Belfast and Kerry," she said.

Ms Ní Riada also references reports that claim that Mr Higgins has been "using the OPW to maintain the garden and grounds" of his private residence.

"These follow other unanswered questions in relation to the use of the €300,000 expense account," she said.

"It is my strong view that discussing these issues publicly is the only means left to reassure our citizens that public finances are being properly used, particularly at a time of huge difficulties for many families.

"These are very legitimate questions, which should be answered in advance of tomorrow night's debate so that we can move on and talk about the next seven years for our country," added the Sinn Féin candidate.

Meanwhile, Mr Higgins has said he follows "the advice of his office" when it comes to travel decisions.

In line with a statement over the weekend, he cited "logistics and security advice" for the decision to fly from Dublin to Belfast. 

When questioned further, he declined to give any more details about what the advice was or who provided it saying: "I have answered the question now - I follow the advice of my office."

Regarding Peter Casey’s decision to remain in the presidential race, Mr Higgins said he had "no comment". 

Mr Higgins was giving a speech in Drogheda about a new initiative on Northern Ireland if he is returned to the Áras.

He was endorsed by the former SDLP deputy leader and former deputy First Minister, Seamus Mallon, who said he had "done well" in his term at the Áras.

Mr Higgins said he wanted to "spark an inclusive and respectful discussion" about how the people of the island can "live together". The policy initiative is entitled 'Shared Ireland, Shared Island'.

Independent candidate Seán Gallagher said Mr Higgins' comments on expenses raised more questions than they answered.

"Why would the President need a Lear Jet to go 100 miles up the road to Belfast to give a lecture?" he asked.

Gavin Duffy accused Mr Higgins of what he termed "Lear Jet socialism". 

Joan Freeman said she wanted the debate to move on from expenses and flights and said her focus was on mental health and well-being.

Mr Casey, who returned to the campaign trail today, said he was focusing his attention on how to help middle Ireland and the people who got little or nothing from the recent Budget.