The group describes itself a voice for reform within the Catholic Church.
It is calling for abuse to be tackled, for LGBT members to be made welcome and for women priests.
Spokesperson Brendan Butler said it hopes Pope Francis could end injustices within the church.
Blue ribbons were tied on the bridge over the River Liffey in a symbol of solidarity with victims and survivors of clerical wrongdoing.
The distinctive pedestrian bridge was also awash with rainbow flags and purple umbrellas supporting the ordination of female priests.
One veteran campaigner also said the Vatican's files should be opened to prove it is serious about tackling child abuse.
Soline Humbert, 62, from Versailles, near Paris, said: "Unless the truth comes out, and we know that as Christians, and we know that as Catholics, there is no movement forward, there is no resurrection, there is no transformation and trust cannot be re-established until the truth is acknowledged.
"It is very painful and it will be very disturbing but the truth is buried in the bottom, in the secret archives, of a lot of dioceses and especially in the Vatican."