Almost 2,000 complaints were received by the Garda Síochána Ombudsman Commission (GSOC) last year.
There were also 38 referrals from the Gardaí of matters where it appears 'the conduct of a member of the Garda Síochána may have resulted in the death of, or serious harm to, a person'.
GSOC said 15 of these referrals related to fatalities.
Its annual report has been put before the Houses of the Oireachtas.
It shows there were 1,921 complaints received - a 1.4% decrease on 2017.
There were 2,944 allegations contained within those complaints - with 1,897 complaints containing 3,949 allegations closed.
The report also said there were 17 public interest investigations - investigations undertaken in the absence of a complaint or referral by the Garda Commissioner.
While 17 files were referred to the Director of Public Prosecutions (DPP), resulting in four directions to prosecute, nine directions for no prosecutions and four decisions pending at the end of 2018.
There were also 24 protected disclosures made to GSOC by members and/or employees of An Garda Síochána.
Seventy-four sanctions were imposed by the Garda Commissioner on individual Gardaí following complaints to and/or investigations by GSOC, and 415 criminal investigations were opened.
The most common matters complained of were abuse of authority, neglect of duty, discourtesy, and non-fatal offences: allegations of a criminal offence listed in the Non-Fatal Offences Against the Person Act 1997 - for example assault, harassment or false imprisonment.
GSOC also made a number of recommendations to An Garda Síochána relating to systemic or management issues, rather than behaviour of individuals.