The Hollywood star wrote to the Sunday World newspaper urging his countrymen to "make history" and vote in support of same-sex marriage at the referendum next spring.
Colin Farrell's older brother Eamon was beaten at school for being gay
Colin said he was "surprised and intrigued" when, aged 12, he found out that his brother was gay.
"I was curious because it was different from anything I'd known or heard of and yet it didn't seem unnatural to me.I just knew my brother liked men and, I repeat, it didn't seem unnatural to me."
"My brother Eamon didn't choose to be gay. Yes, he chose to wear eyeliner to school and that probably wasn't the most pragmatic response to the daily torture he experienced at the hands of school bullies.
"But he was always proud of who he was. Proud and defiant and, of course, provocative."
Describing Eamon and his partner artist Steven Mannion as "the healthiest and happiest couple I know", the 38-year-old actor said they had travelled to Canada to exchange vows.