The Workplace Relations Commission has ordered Daft.ie to refrain from publishing and displaying discriminatory property ads on its website.
According to the WRC, Daft breached anti-discrimination legislation by allowing certain types of ads to be be published on the site.
The Irish Human Rights and Equality Commission (IHREC) had taken a case against the property site.
The IHREC said it had identified a number of ads in 2016 that discriminated on the housing, age and family status grounds of the Equal Status Acts.
They highlighted the use of phrases such as 'suits professionals only', 'would [not] suit family, children, single person, couple' or 'no rent, rental supplement or allowance accepted... no housing assistance accepted' in ads posted by Daft users.
Daft, meanwhile, argued it was not an advertiser but instead an Information Society Service Provider (ISSP).
The company also said they had already taken a number of measures to help advertisers comply with their obligations under discrimination laws.
In a decision, the WRC ruled in favour of the IHREC - finding Daft has a "vicarious liability" for ads posted on the site.
Adjudication officer Orla Jones said: "I conclude that the complainant has raised a prima facie case in respect of discrimination on the grounds of family status and age and on the rent allowance ground which the respondent has failed to rebut.
"I am also satisfied that the respondent in permitting these advertisements to be displayed on its website has breached Section 12 of the Equal Status Acts."
Daft is now being ordered to refrain from publishing or displaying "advertisements that indicate an intention to engage in prohibited conduct".
The WRC says the measures to achieve that should include a filter identifying “trigger” words and phrases that could be in contravention of equality legislation.
In a statement, the IHREC welcomed the WRC ruling.
Commission member Tony Geoghegan said: "In the context of the current crises in homelessness and housing, this ruling marks a significant step in efforts to curtail discriminatory advertising particularly around rental accommodation.
“The Commission is seeing systemic discrimination against people in receipt of housing social welfare payments, against people with families, against those seeking employment and against young and old people.
"Advertisements for accommodation should describe the property available, not people."
He added: "Today’s outcome cuts off advertising options for people who would seek to discriminate when advertising rental accommodation."
