The party was founded by Peadar Tóibín, a TD who resigned from Sinn Féin on 15 November 2018 due to his anti-abortion views after opposing the party whip on the Health Act 2018. He began canvassing elected representatives, securing support within a week from two local councillors in the Republic. Tóibín held meetings across the island addressing interested potential members. The first Northern Ireland local councillor declared on 7 January 2019. eight councillors had joined. A second councillor in Northern Ireland joined on 26 February 2019. The name Aontú was announced at a meeting in Belfast on 28 January 2019. The Meath Chronicle said that the announcement of the name was precipitated by its unexpected publication on the UK Electoral Commission website. Tóibín said the party had sought registration in both jurisdictions, that "Aontú obviously means unity and our major objective is the unity of Irish people north and south". He recalled that Belfast was the birthplace of the United Irishmen of 1798. Aontú would "seek to build an all-Ireland economy to mitigate the worst effects of Brexit, economic justice for all and to protect the right to life." Tóibín said he was talking with Sinn Féin, SDLP and independent representatives in Northern Ireland, and that "people from Sinn Féin, SDLP and Fianna Fáil backgrounds would feel comfortable" in the party.
At the 2019 Northern Ireland local elections on 2 May, Aontú nominated 16 candidates across 7 of the 11 local councils. It won one seat, Anne McCloskey in Ballyarnett DEA on Derry and Strabane Council, with its two outgoing councillors losing their seats. Several months after the election, a councillor for the SDLP in Mid Ulster joined Aontú. The party nominated 53 candidates for the Republic's local elections on 24 May, including its seven sitting councillors. Three were elected. It did not run any candidates in the elections for the European Parliament held in Northern Ireland or in the Republic.
2019 Dáil by-elections
Aontú contested two of the four 2019 Dáil by-elections. Finian Toomey came 7th in Cork North-Central with 1,008 votes, being eliminated on the fifth count. Jim Codd came 6th in Wexford with 2,102 votes, being eliminated on the second count.
Aontú fielded 25 candidates in the 2020 Irish general election, including leader Peadar Tóibín, deputy leader Anne McCloskey and a number of sitting local councillors. Tóibín was the only successful candidate. As Tóibín was not invited to participate in a televised debate alongside the leaders of other parties, the party threatened a High Court action against RTÉ. The party, however, did not proceed with the action noting that there "was not enough time to have the action heard" before the debate. In the 2020 Seanad election, Paul Lawless contested the Cultural and Educational Panel receiving 2.6% of votes.
