Eamonn and Steven - who spoke exclusively to Attitude about the campaign for equality in Ireland earlier this year - plan on finally marrying in Ireland next year, and Colin has told the Irish Independent that he will "definitely" be returning home to serve as his older brother's best man.
He continued: "I have to talk to him about when that is. We have to organise it because so many of the family are over here [in LA] now."
The True Detective star added that he regretted not being in Ireland to enjoy the celebrations following the vote count back in May. He previously mocked the Catholic Church's negative reaction to the result- click here to find out more.
"If I'm honest, selfishly I'm sick I wasn't at home," the 39 year-old said. "It nearly took the joy out of it for me."
He went on: "I was happy for everybody 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."