The Sinn Féin leader says President Michael D Higgins is 'correct' not to attend an event marking the partition of Ireland, labelling it a 'catastrophe'.
Mary Lou McDonald was speaking as controversy continues over President Higgins declining of an invite in October.
President Higgins has denied his not attending is a snub to Britain's Queen Elizabeth II.
He says he declined the invite because the event had become politicised - while he was incorrectly referred to as 'President of the Republic of Ireland' rather than the President of Ireland.
The church service to mark the centenary of the partition of Ireland is due to take place in Co Armagh on October 21st.
In a tweet, Ms McDonald says she supports the President's 'clear' decision.
"The partition of Ireland was a catastrophe for our people and our country.
"The partition of Ireland costs us to this day, holds us back, divides us.
"Uachtarán na hÉireann has made clear his decision not to attend a commemorative event. His decision is the correct one."
The partition of Ireland was a catastrophe for our people and our country. The partition of Ireland costs us to this day, holds us back, divides us. Uachtarán na hÉireann has made clear his decision not to attend a commemorative event. His decision is the correct one.
— Mary Lou McDonald (@MaryLouMcDonald) September 17, 2021
While former Taoiseach John Bruton has said he believes the President 'is wrong' not to attend the event.
He earlier told Newstalk Breakfast: "I think he is wrong not to attend, and I think there is time for him to change his mind.
"Obviously it requires great strength of character to change one's mind, but I think he should do so.
"This is not celebrating partition, it is simply recognising a fact.
"And it is important to recognise that fact - the fact that Northern Ireland exists and is legitimate - was recognised by the Irish people when they voted in the referendum on the Good Friday Agreement.
"So in going to this event and recognising that Northern Ireland existed for the last hundred years, the President would simply be recognising something that the Irish people have recognised."
While Foreign Affairs Minister Simon Coveney says President Higgins "made his decision on this" and that should be respected.