At least 20 officials and staff at the department were pictured celebrating with champagne in an event at Iveagh House on June 17 2020 while pandemic restrictions were still in place.
A spokesperson said steps were taken to ensure “lessons have been learned” following the breach of public health guidelines.
The party took place after the country secured a seat on the UN Security Council following a 15-year campaign.
In a statement, the department said: “The Department of Foreign Affairs security council campaign team was working in a large open plan area of Iveagh House throughout the day of June 17 2020.
“Had Ireland not won the council seat on the first round of voting, the team would have had to work through the night to campaign for a second vote the next day.
“As it happened, we won in the first round and, in a moment of happiness, as the result came in, we briefly let our guard down by celebrating together.
“It was 18 months ago and steps have been taken to ensure lessons have been learned.”
Strict lockdown controls were still in place at the time of the party, with indoor gatherings of more than seven people prohibited.
People were unable to travel outside their own counties, funeral attendances were limited to just 25 people and the public had been urged to work from home where possible.
The “selfie” photo was taken by the then-secretary general at the department, Niall Burgess, who has since taken up the role of Ambassador to France.
Also featured are John Concannon, who worked on the UN Security Council campaign and was once the head of the controversial strategic communications unit under taoiseach Leo Varadkar in 2017.
Mr Concannon has since moved on to become director general of the department’s Global Ireland division.
Brendan Rogers, the then-deputy secretary general at the department, who went on to become Ambassador to the Netherlands, also appears in the photo.
Independent TD Michael McNamara has criticised the department for “hypocrisy”.
He wrote on Twitter: “The same department ceased processing passports applications during 2020 (without any lawful basis) to enforce lockdown.
“Maybe hypocrisy is an inevitable part of such a hysterical response.”