[1] He served as Deputy Lord Mayor of Dublin in 2006, during which time he launched a Right to Read Campaign in an effort to improve the poor literacy rates in disadvantaged areas.

[9][10][11] As Minister of State he prioritised reforming the direct provision system, ending the legal entitlement of Church-controlled state-funded institutions to discriminate against LGBT people, and played a key role in the Yes campaign in the 2015 marriage equality referendum.

[citation needed] On 22 April 2015, Ó Ríordáin was appointed as Minister of State at the Department of Health, with responsibility for the National Drugs Strategy, in addition to his existing ministerial duties.

[15] After Brendan Howlin was elected unopposed as Labour leader, he appointed Ó Ríordáin as Spokesperson on Environment and Sustainable Development, and Gaeltacht Affairs.

On 10 November 2016, following the United States presidential election, Ó Ríordáin made a public statement in the Seanad that went viral on social media, in which he labelled President-elect Donald Trump as a "fascist" and a "monster", quoting Edmund Burke's attributed maxim that "the only way evil can prosper is for good men to do nothing".

He condemned Trump's statements threatening to imprison his opponent, former Secretary of State Hillary Clinton, barring Muslims from entering the country, mass deportations, and his assertions that the media and the political system were rigged.

He applauded SDLP leader Colum Eastwood's statement that his party would boycott the St. Patrick's Day ceremony at the White House during Trump's presidency.

Ó Ríordáin was also publicly supported by former Labour TD Liz McManus and former lord mayor of Dublin Andrew Montague.

[22] In December 2023, Ó Ríordáin announced that he would seek the Labour Party nomination to run in the 2024 European Parliament election in Dublin.

In an interview with the Irish Independent, Ó Riordáin said he had initially decided not to contest the election, but had a "visceral" reaction following the violence in the 2023 Dublin riot.

[31] In an opinion piece in TheJournal, Ó Riordáin criticised the "war on drugs", mentioning a case where a young man was charged in court with possession of €2 worth of cannabis, describing it as a "waste of Garda time".

[36] As Labour's spokesperson on education, Ó Riordáin has called for single-sex schools to be abolished,[37] saying that they are "part of the problem" of gender inequality.

[38] He branded education minister Norma Foley and the government a "bad debs committee" in the Dáil over their ruling on mask-wearing in primary schools.

In a TV appearance on RTÉ's Saturday Night Show during the campaign, Ó Ríordáin was asked by presenter Brendan O'Connor to remove a rainbow "Yes" pin from his collar.