Northern Ireland Deputy First Minister Martin McGuinness will meet Britain's Queen Elizabeth for the first time next week.
The meeting and handshake will take place in private.
Sinn Féin's Ard Comhairle met in Dublin today to discuss whether Mr McGuinness should attend a function in Belfast, which will be attended by the Queen.
Party leader Gerry Adams said the lunch was not connected to the Queen's jubilee celebrations and said the Ard Chomhairle had agreed in the context of national reconciliation that the invitation could be accepted.
Mr Adams confirmed that Sinn Féin would observe the "civic niceties" at the function.
He acknowledged that the decision would cause difficulties for Republicans, but said Sinn Féin remained committed to nation building and ending partition.
Mr Adams said it had not been a unanimous Ard Comhairle decision but there had been a healthy majority.
"This will understandably cause difficulties for some republicans and nationalists," Mr Adams said.
"Especially for those folks who suffered at the hands of British forces."
Co-Operation Ireland is involved in reconciliation activities on both sides of the border.
Its joint patrons are President Michael D Higgins and Queen Elizabeth.
Mr Adams said Sinn Féin wanted to see a new republic in which the traditions of orange and green could be brought together in a cordial union.
Mr McGuinness declined the invitation to attend a State dinner at Dublin Castle when the Queen made her first state visit to the Republic of Ireland last year.
