As part of his tour of the nations of the UK, Britain's King Charles III and Queen Consort Camilla were at Hillsborough Castle in Co Down on Tuesday.
The new King attended a service of reflection for Britain's Queen Elizabeth in St Anne's Cathedral in Belfast.
Among the people attending the service and meeting the King and Queen Consort were President Michael D Higgins and Taoiseach Micheál Martin.
Earlier on Tuesday, the King met the leaders of the two biggest parties in Northern Ireland in the form of DUP chief Jeffrey Donaldson and Michelle O'Neill, the Sinn Féin leader in Northern Ireland.
During their brief conversation, the new King thanked Ms O'Neill for the "incredibly kind things" she had said about his mother.
Last week, Ms O'Neill issued a statement following the Queen's death saying she was "grateful for Queen Elizabeth's significant contribution and determined efforts to advancing peace and reconciliation" and that she "led by example."
Responding to the King's gratefulness, the Sinn Fein vice president said: "She played a great role here in terms of reconciliation...It's the end of an era for sure."
But the new King has clearly been keeping tabs on Northern Ireland, as he noted to Ms O'Neill: "What are you now, the biggest party, are you?"
"We are indeed, we are indeed," O'Neill replied.
Sinn Fein did gain the most votes during May's election, but there is still no Government in Stormont at time of writing as the DUP have refused to assent to the election of a Speaker as part of their protest against the Northern Ireland Protocol.
Mr Donaldson said of King Charles III: "He has demonstrated a very sharp insight into Northern Ireland and shown a great interest in the development of Hillsborough Castle,"
"He is sensitive of political differences here and keen to build on reconciliation.
"There are lots of people here today, here to welcome their new head of state.
"There is a lot of interest in the monarchy here in Northern Ireland."
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