Vigils are taking place around the country to remember Aidan Moffitt and Michael Snee.
The two men were murdered in their homes in Co Sligo this week in suspected homophobic attacks.
Over 20 events are being held across the country, with more vigils set to take place on Saturday.
Dublin Lord Mayor Alison Gilliland told those gathered outside the Dáil that diversity gives the capital its character.
"For people like us here, showing our solidarity.... it is really, really important and it's incumbent on us all that we are pro-active, visible, strong allies.
"That we call out homophobic, biphobic and transphobic language and behaviours.
"We need to overtly stand with and stand for those who may feel fear in our city.
"Today we are showing that - but we also need to show that within our families, within our workplaces, within our places of learning and in our clubs and recreation facilities.
"It's up to us, all of us here, to be those allies: show our strength, and show our absolute disrespect for people who disrespect others."
James O'Hagan of LGBT Ireland says the gatherings send a strong message.
"The most significant statement being made here this evening, and at vigils all around the country, is yours.
"By coming here and standing in solidarity with the LGBTI+ community, you tell those who would discriminate against us that their actions are wholly unacceptable.
"This is a space to grieve, express your sorrow and anger, and also a space to acknowledge that the events of the past week - including the horrific attacks on Evan Somers in Dublin last weekend and a third victim of the attack in Sligo - have retriggered a fear of which all members of the LGBTI+ community have felt in our daily lives."