The tricolour will fly at half mast on all State buildings today to mark the death of Britain's Prince Philip.
The funeral of the Duke of Edinburgh, who died on April 9, will take place today.
The Queen and just 30 guests have been invited to the service due to Covid restrictions limiting mourners.
The Irish national flag will fly at half mast for the entire day to mark the passing of the prince.
A Government spokesperson confirmed: "As a mark of respect to His Royal Highness, Prince Philip, Duke of Edinburgh, the national flag will be half-masted on all State buildings equipped with a permanent flagpole, on the day of his funeral, 17 April 2021."
This is not the first time the flag has been lowered in memory of members of the extended British monarchy — in 1997, the tricolour flew at half mast on all State buildings on the day of Princess Diana's funeral.
This was also done in 2002 to mark the death of the British Queen Mother.
Section 8 of the National Flag Guidelines states that “on the death of a national or international figure, the National Flag is flown at half-mast on all prominent government buildings equipped with a flag pole, under advice from the Department of the Taoiseach."
The Department of the Taoiseach can also request to have the flag flown at half mast to mark other tragic events.