The Sinn Féin leader Mary Lou McDonald has 'unreservedly apologised' that robust procedures were not in place when Mairia Cahill reported abuse to the party.
An investigation of a complaint about how the PSNI handled reports of alleged child abuse found police there failed those who were said to have been abused.
The PSNI Chief Constable George Hamilton has also apologised unequivocally for "hurt and distress" caused to Ms Cahill and two other women failed by police.
The investigation, by the Police Ombudsman's Office in Northern Ireland, also found that the PSNI's predecessor - the RUC - had information about the alleged abuse 10 years earlier but did not investigate it.
In 2010 Mairia Cahill, who is a member of a prominent Republican family, told police she had been sexually abused by a prominent IRA member from 1997 to 1998, and in subsequent years was subjected to an IRA 'investigation' of her allegations.
Two other women also said they too had been abused as children by this man, named in the report.
Mr Hamilton says: "At the heart of this report are three victims abused as children, who were then failed by their police service.
"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."
"Whilst PONI has stated that they are satisfied that current police practices would not allow such information to go un-investigated today, the report noted other failures in the PSNI's investigations.
"These standards fall very short of the high expectations that I and my officers set ourselves and that the public expect."
File photo of PSNI chief constable George Hamilton | Image: Brian Lawless/PA Wire/PA Images
"I accept the report and we have since implemented all the recommendations made by PONI for changes to PSNI policies."
The PSNI began investigations into both these matters, which resulted in a number of people being prosecuted.
In 2014, the trials of the man accused of the rape and of those accused of involvement in the IRA investigation collapsed when Ms Cahill and the two other women withdrew their evidence, citing a loss of confidence in how the matter had been dealt with.
In 2015, a review by Keir Starmer found the North's Public Prosecution Service had failed the women.
Ms Cahill also made a series of complaints to the Police Ombudsman's Office about the PSNI's handling of her report to them.
Police Ombudsman Dr Michael Maguire has met her to discuss his investigation's findings.
Speaking to Newstalk Breakfast, Ms Cahill says: "I'm incredibly upset but also there's a bit of relief that it, as far as I'm concerned, proves that I was telling the truth.
"But also when I got the 65 page disclosure letter - it was given to me yesterday - I felt sick to my stomach reading it, to be quite honest.
"I haven't quite digested all of it yet - I'm going to take the next bit of time to do that".
Mairia Cahill says the report "absolutely proves" her account | Image: Sam Boal/Rollingnews.ie
"What the letter actually says is that in the course of his investigation, the Ombudsman discovered intelligence which had been given separately to both Special Branch and CID - which was the Criminal Investigations Department of the RUC then - to say that not only were people being abused by my abuser, or suspected of abuse, but that there was a Provisional IRA investigation into that abuse.
"And further, what isn't contained in the press release today, is that other intelligence was received in 2000 to say that Sinn Féin had suspended this particular man as a party member because he was suspected of abusing children.
"Now what is crucial around that... is that I first disclosed my abuse to senior Sinn Féin members in 1997 and '98 while it was still happening - and it took them till August 2000 to do anything about it".
Ms Cahill says she wants an apology from the party and its leader, Mary Lou McDonald.
"I think first of all she owes me an apology.
"Secondly, she has to admit now at this stage - Sinn Féin are very fond of waving Ombudsman reports around when it suits them - but what she needs to admit now is to certainly to accept that in the course of the Ombudsman investigation that he obtained contemporaneous evidence in the form of intelligence which was provided which, as far as I'm concerned, absolutely proves and categorically proves my account".
Sinn Féin President Mary Lou McDonald speaking to the media in Dublin | Image: Sam Boal/RollingNews.ie
In a statement, Ms McDonald says: "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."
