Byrne worked as a solicitor at Feran & Co. law firm in Drogheda until his selection as a Fianna Fáil 2007 Irish general election candidate for the newly established constituency of Meath East.

[6] In his first campaign for public office, Byrne was elected to the Dáil with 7,834 first preference votes,[7] making him the youngest member of the Fianna Fáil parliamentary party at the time.

[12] In June 2009, following selection to run alongside the existing Fianna Fáil MEP Liam Aylward, Byrne unsuccessfully contested the 2009 European Parliament election for the East constituency receiving a total of 33,383 votes.

[16] In May 2014, following selection to run alongside the sitting Fianna Fáil MEP Pat "the Cope" Gallagher, Byrne unsuccessfully contested the European Parliament election for the Midlands–North-West constituency receiving a total of 64,057 votes (8.6%).

[19] As opposition spokesperson for Education and Skills, Byrne opposed so-called 'baptism barrier' which was instituted in oversubscribed Irish Catholic primary schools.

The practice, in line with Catholic primary schools admissions policies at the time,[20] involved the lawful discrimination of student registration on the basis of religion.

[22] However, Fianna Fáil Senator Lorraine Clifford-Lee lost the election to Green Party candidate Joe O’Brien TD.

[28] After five months of discussion, held during the COVID-19 pandemic, Fianna Fáil, Fine Gael and the Green Party reached an unprecedented agreement to coalesce and establish a governing majority.