In a statement issued yesterday, Mrs Higgins said the letter was written in a “personal capacity”.
She said she put the letter on her dedicated section on the President’s official website so that people could access it.
However, she said she soon took it down when she saw it “being presented as not being from myself, but from the general President.ie website”.
“For the last 11 years since coming to Áras an Uachtaráin as the wife of the President, I have continued my long-standing interests on a number of important issues,” she said.
“Since 2014, I have had a dedicated section on the President.ie website.
“This section contains details of activities I have been undertaking including speeches and work towards the implementation of the United Nations’ sustainable development goals, health issues, breastfeeding, issues affecting the Traveller Community, human rights, supporting the arts and a range of other issues,” she said.
“Last week I had been asked about my letter to the Irish Times, which I had written in a personal capacity, by a number of people who had missed it, and had not been able to access it online.
“I therefore put it on my dedicated section of the website as I have done for the last number of years. Having put my letter up, I subsequently took it down when I saw it being presented as not being from myself, but from the general President.ie website.
“I have from its outset strongly condemned the illegal Russian invasion of Ukraine and I cannot be but dismayed that people would find anything unacceptable in a plea for peace and negotiations when the future of humanity is threatened by war, global warming and famine.”
It follows days of mounting pressure over the letter and its publication on the website.
In the letter, Mrs Higgins said the fighting in Ukraine would go on until the world “persuades President Vladimir Putin of Russia and President Volodymyr Zelensky of Ukraine to agree to a ceasefire and negotiations”.
Fianna Fáil senator Malcolm Byrne, who previously took issue with the contents of the letter and its publication on the presidential website, yesterday welcomed Mrs Higgins’s statement but said it should have been published earlier.
“If this had been issued last week when this controversy first arose, it would have made matters clearer,” he said.
“Unfortunately, in the meantime, the Kremlin did use her letter for propaganda purposes and she could have made clear earlier that this was not acceptable,” he added.
Mr Byrne said Mrs Higgins’s response could have been issued alongside the President’s comments on the controversy on Monday.
“It is important that we all seek peace but that Russian aggression and war crimes continue to be called out,” the Wexford senator said.
Meanwhile, Independent senator Gerard Craughwell said he “can see no justification for any private citizen having a dedicated section” on the President’s official website.
“We need to clarify the role (if any) of the spouse or partner of any sitting president.
“We still need to hear from the President himself,” he added.
In a statement issued on Monday, a spokesperson for the President said Mr Higgins has repeatedly condemned the “illegal, immoral and unjustifiable Russian invasion of Ukraine”.
However, that statement made no mention of why the letter was published on the President.ie website.