Northern Ireland's nationalist first minister-elect on Wednesday said she will attend King Charles III's coronation, despite her party's traditional opposition to the monarchy.

"I have accepted an invitation to attend the coronation of King Charles III," Sinn Fein vice-president Michelle O'Neill said in a statement.

Sinn Fein, which wants reunification on the island of Ireland, is the former political wing of the IRA paramilitary group that fought against British rule in Northern Ireland until a peace deal in 1998.

It was notably responsible for planting a bomb that killed Charles's mentor, Queen Elizabeth II's cousin Lord Louis Mountbatten, in 1979.

Sinn Fein MPs elected to the UK parliament in London have long refused to take an oath to the British monarch, and cannot sit in the chamber.

But there have been signs in recent years of a change in approach, as Sinn Fein has gained more popular support and with O'Neill in line to be first minister in Northern Ireland's government.

Last year, O'Neill publicly praised Queen Elizabeth II for her role in cementing the peace process with a landmark 2011 trip to Ireland. 

The party's president, Mary Lou McDonald, the previous year also apologised for the first time for the IRA attack on Mountbatten.

On Wednesday, O'Neill said: "We are living in a time of great change.

"A time to respect our differing and equally legitimate aspirations, a time to firmly focus on the future and the opportunities that the next decade will bring.

"I am an Irish Republican. I also recognise there are many people on our island for whom the coronation is a hugely important occasion.

"I am committed to being a first minister for all, representing the whole community, building good relations between the people of these islands, and advancing peace and reconciliation through respectful and mature engagement."

Also attending the May 6 coronation will be Ireland's President Michael Higgins, church and political party leaders from Northern Ireland, she added.