O'Callaghan has presented Prime Time since 1996, and her own summer talk show, Saturday Night with Miriam, from 2005 onwards.

[2] Her brother, politician Jim O'Callaghan is a Fianna Fáil TD[3][4] and since 24 January 2025 has served as Minister for Justice (Ireland).

[6] O'Callaghan worked for British national broadcaster the BBC[7] before returning to Ireland after being headhunted by RTÉ in 1993 to present Marketplace, an economics and business programme.

[12] In 2013, she stood in as a long-term replacement on RTÉ Radio One in the daily morning slot from 9 am to 10 am when presenter John Murray was on extended sick leave.

In 2011, O'Callaghan fronted RTÉ's coverage of Queen Elizabeth II's state visit to the Republic of Ireland in May 2011,[13] and in October of that year hosted the Prime Time debate with the seven candidates standing in the 2011 Irish presidential election.

[15] She hosted RTÉ's 50th anniversary party on New Year's Eve 2011, attended by President of Ireland Michael D. Higgins and Jedward.

RTÉ defended O'Callaghan against accusations of bias, her brother was standing for re-election as a Fianna Fáil candidate.