On leaving school, he worked as a clerk in the Special Detective Unit in Dublin and in 2000, he enrolled in the Garda Síochána (Ireland's National Police Force).

[13] O'Reilly also infiltrated 'Gangland Limerick' whilst living as a heroin addict,[14] in one of the most high profile long-term undercover drugs operations in An Garda Siochana's history.

[15][16] He was also involved in the seizure of €29 million worth of cocaine which resulted in lengthy prison sentences for both Gareth Hopkins and Abraham Shodiya.

[22] The Sunday Times journalist Liam Fay wrote: "...but its initial believability has been enhanced by a strikingly rough-edged performance from Kieran O'Reilly, a real-life undercover Garda".

O'Reilly's character met his demise in what has been described as the best one-on-one fight in the history of the show, when 'White Hair' took on lead actress Katheryn Winnick who played 'Lagertha', in what would be 'Lagertha's' last battle before being killed herself by 'Hvitserk'.

During O'Reilly's time in the band, they were selected by Hot Press Magazine to have a music video shot for "Will I Ever Learn" in conjunction with the Tisch School of the Arts, New York University.

[36] White McKenzie also opened the Kildare leg of the First Music Contact Tour on 9 November 2012, sharing the stage with Squarehead, We Cut Corners and The Lost Brothers.

[41][42] In 2014 O'Reilly formed Hail The Ghost which includes two former White McKenzie members, Ian Corr (piano/keys) and Eamon Young (guitars).

He credits The National, The Antlers and Elbow as influences when he was composing the album's songs which would be: "a piece that would have that cohesive feel to it, we would have that thematic thing and we would also have a story, beginning to end.

[47] Hot Press Magazine reported: "Last time round, we described them as "atmospheric indie", but 'Arrhythmia' is composed of bleaker soundscapes, more reminiscent of Joy Division".

[48] On 5 February 2020, Kieran O'Reilly released "Broken Wings" with his on-screen Vikings co-star and former double Olympian, Ragga Ragnars.

[49] The music video for the track was filmed on location in Iceland by Ragnars' brother and filmmaker, Gudjon Ragnarsson.

[57] O’Reilly joined Mount Tallant Boxing Club in 1996 where he was trained by former double Olympian, Mick Dowling.

In 2002, O’Reilly was defeated by Jamie Power of St. Francis Boxing Club in the quarter-finals of the National Irish Junior Championships.

In 2003, O’Reilly won gold at the World Police and Fire Games in Barcelona in the Intermediate Light-Heavyweight division.