Sinn Féin leader Mary Lou McDonald has 'unreservedly apologised' that robust procedures were not in place when Mairia Cahill reported abuse to the party.
Ms Cahill told police in 2010 she had been sexually abused and was also subjected to an IRA 'investigation' into her allegations.
She says that Sinn Féin allowed people to believe she wasn't credible.
The Police Ombudsman of Northern Ireland found a PSNI investigation failed the Belfast woman and two others.
Mairia Cahill says the report "absolutely proves" her account | Image: Sam Boal/Rollingnews.ie
Ms McDonald has welcomed the report, and commended Ms Cahill's bravery.
She said: "I welcome the publication of the Ombudsman's report and the fact that the PSNI have accepted and will implement the recommendations of that report.
"Abuse has scarred too many lives across Ireland. We all have a responsibility to keep children safe.
"I have no doubt that the three women at the heart of this report have been through an ordeal.
"I want to commend their bravery, in particular the bravery of Mairia Cahill for waiving her anonymity.
"Sinn Féin has robust procedures in place for mandatory reporting of abuse.
"I deeply regret that these procedures were not in place at the time of Mairia Cahill's disclosure. For this I unreservedly apologise.
"I wish Mairia Cahill every best wish for the future."
Earlier, PSNI Chief Constable George Hamilton apologised unequivocally for "hurt and distress" caused to the three women failed by police.
Mr Hamilton said: "I apologise unequivocally for the hurt and distress caused to them and for the failures in the police investigation.
"The Police Service of Northern Ireland fully supports the Office of the Police Ombudsman. It is an essential part of the mechanisms by which we can be held to account.
"The report found failures by the RUC in 2000, to share vital information which linked a man to the alleged abuse of children."
Three members of the PSNI have been disciplined - while a fourth had retired.