While Sinn Fein lord mayors in Belfast have laid laurel wreaths on the anniversary of the First World War battle in previous years, Friday was the first occasion a member of the party’s leadership has taken part.
Ms O’Neill joined current Sinn Fein Lord Mayor Tina Black to lay a wreath on Friday morning, two hours ahead of the official commemorative event marking the first day of the battle in 1916.
The two republican politicians stood in silence in front of the Cenotaph after laying the wreath for those who died.
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The Sinn Fein Stormont leader, who will be entitled to be nominated as First Minister when and if powersharing is resurrected, said her attendance was proof of her commitment to work for everyone in society.
The Battle of the Somme was one of the largest and bloodiest battles of the First World War.
It saw the 36th Ulster Division and the 16th Irish Division, representing the two main traditions from the north of Ireland, distinguish themselves, but at a great cost.
The battle has assumed major symbolic significance for the unionist community in Northern Ireland.
“I said throughout the election campaign that I wanted to be a first minister for all and I hope that today’s attendance and the laying of a wreath is actually a demonstration of someone who wants to work for all in our community,” Ms O’Neill told reporters on Friday.
“I think as political leaders, we have a responsibility to reach beyond our comfort zones and actually reach out the hand of friendship, and to try to do whatever we can in terms of leadership in terms of healing the wounds of the past.
“So, I’m very pleased to be here this morning to have laid a wreath in terms of all those lives lost.”
Last July, the then deputy first minister joined then first minister Paul Givan of the DUP at the Irish National War Memorial at Islandbridge in Dublin to remember those who fell at the Somme.
That was then the first time the Sinn Fein politician had attended a Royal British Legion wreath-laying ceremony.
Ms O’Neill declined to be drawn on Friday when asked why she was prepared to attend an official event in Dublin but not in Belfast.
“I don’t think we should get distracted from the fact that this is quite significant, the fact that I have laid a wreath this morning, along with our mayor Tina Black,” she said.
“I’m doing so to be respectful, I’m doing so to actually try to reach out the hand of friendship to actually show political leadership. And I think that will not be lost in the wider public.”
She added: “I hope that sends out a strong message that I do genuinely want to lead for everybody in this society.”
Ms O’Neill also was not drawn on whether Sinn Fein would consider attending official Somme commemorations in Belfast in the future.
Asked whether she felt her gestures were being reciprocated by political unionism, she said: “What I’m doing today is not about reciprocation, it’s actually about demonstrating respect and my leadership, and what I’m determined to do, regardless of what others decide to do.”