The Association of Catholic Priests and the Lay Catholic Group have called on Church to restore Tony Flannery to ministry.
Eleven years ago, the Congregation for the Doctrine of the Faith instructed the Redemptorist authorities to remove Fr Flannery indefinitely from his priestly ministry.
The redemptorist priest was silenced by the Vatican for publicly expressing support for women's ordination and same-sex marriage, as well as liberal views on homosexuality.
In a statement, the ACP and the LCG said that by being denied standard prerequisites of a just and fair procedure, a grave injustice had been perpetrated.
They have described the situation as "grotesque".
"There is a growing presumption because he has been sidelined that he is persona non grata. This presents him in a totally unfair, unjust and, above all, false light," they said.
The two organisations have pointed out that Fr Flannery was never informed who his accusers were, never given an opportunity to defend himself and never allowed to appeal the decision.
"This flawed process has condemned Tony Flannery - now in his late 70s - to a gratuitous and unacceptable punishment of banishment from the exercise of his priesthood, effectively for life."
They also pointed out that he was refused permission to officiate at his sister's funeral, as per her wishes.
The ACP and the LCG have questioned how the Church could accept the decision against Fr Flannery, given that the issues he wrote about which resulted in his removal from priestly ministry are now healthily being discussed in the public forums of the Catholic Church as part of the Synodal Pathway.
By restoring Fr Flannery to ministry, the ACP and the LCG say it would give some credence to "the infinite tenderness of God" of which Pope Francis speaks.