The Irish actor has voiced his disappointment about not being in Dublin for the same-sex marriage referendum result.ABC TV

Irish actor Colin Farrell is set to stand as his brother’s best man at his upcoming Irish wedding ceremony.

Colin’s brother Eamon married his partner Stephen Mannion in Canada in 2009 and held a civil partnership ceremony in Ireland, for which Colin also acted as best man.

They now plan to hold another ceremony in Ireland this year, however, celebrating the historic referendum vote in favor of same-sex marriage and the actor will once again stand by his brother’s side.

Farrell has also spoken of his disappointment that he was not in attendance at Dublin Castle when the historic result was announced and celebrated nationwide.

"If I'm honest, selfishly I'm sick I wasn't at home. It nearly took the joy out of it for me," he said.

"I was happy for everyone else, but I was looking at the pictures online of Dublin Castle and was sick. I think in life sometimes you just want to be around good stuff."

"You want to be around people that are up for the craic and kind, that you feel you can lean into and that you're OK with them leaning into you."

Farrell has previously been very outspoken about LGBT rights, especially during the Yes campaign for same-sex marriage in Ireland.

Last year, he penned an open letter in support of his brother and a Yes vote in the referendum. In the moving letter, he stated, “How often do we get to make history in our lives? Not just personal history. Familial. Social. Communal. Global. The world will be watching. We will lead by example. Let's lead toward light.”

The star of the second season of “True Detective” published the letter in the Irish Sunday World, outlining the challenges, bigotry and hate that his brother Eamon had dealt with all of his life and of the pride he feels for Eamon in facing all of those challenges with bravery.

“Even when others were casting him out with fists and ridicule and the laughter of pure loathsome derision, he maintained an integrity and dignity that flew in the face of the cruelty that befell him,” Farrell wrote.

“The fact that my brother had to leave Ireland to have his dream of being married become real is insane.”

There are still currently two appeals against the referendum outcome to be heard in the Dublin Court of Appeals this Thursday before the result can officially come into law throughout the country.