It has been revealed that a garda was reminded 15 times to progress an investigation into a rape allegation but it was not "acted upon".
The case is among almost 2,000 complaints made to the Garda Síochána Ombudsman Commission last year.
In its 2018 Annual Report, GSOC said a woman complained to the commission about "a failure to investigate" an alleged rape that she reported to two gardaí.
GSOC found that the investigating officer "took a statement from the woman but did nothing further" with it.
The officer maintained that "the matter was referred to the detective branch for further investigation but no documentary proof of this referral was found" by GSOC.
The commission said "the records did show that the investigating garda received 15 separate reminders to progress the allegation but none of the reminders were acted upon".
GSOC recommended that the Garda Commissioner should take disciplinary action against the investigating garda, after it found there was sufficient evidence to indicate a breach of discipline.
The report said the garda was fined.
It was recommended by GSOC that no disciplinary action be taken against the second garda, who was also the subject of the complaint.
The commission found the second garda "was present when the statement was taken from the investigating officer but had no further dealings with the case and was unaware of the lack of progress".
The 1,921 complaints received by GSOC in 2018, represented a fall of 1.4% on the previous year.
Abuse of authority, neglect of duty and discourtesy were among the most common issues complained about.
74 sanctions were imposed by the Garda Commissioner on gardaí following complaints and investigations by the commission.
GSOC also revealed that 24 protected disclosures were made by members or employees of the Garda Síochána during 2018.