[5] Sheehan attended St Paul's school in Portlaoise after the closing of Ballyfin College, which went on to become a famous hotel.

Unsure of whether acting was a sustainable career choice, he studied film and television at Galway-Mayo Institute of Technology.

He missed several months of the course to film Summer of the Flying Saucer, failed his first-year examinations, and decided not to attempt the repeats.

At the age of fourteen, he auditioned for Song for a Raggy Boy, and won the role, spending three months in Cork to film.

[13] Regarded as a "cult favourite",[8] Sheehan connected "intensely" with the role,[6] which he later described as "defining", and "a lovely, gorgeous memory for me,"[14] and remarked it was "challenging" to leave it behind.

[12] After leaving the series, his character's ubiquitous blue hoodie was donated to the Dunamaise Theater in Portlaoise, and he remarked it was "tough to watch [the] show carry on without [me]".

[18] The following year, he starred in the film Killing Bono,[19] and in John Crowley's production of The Playboy of the Western World at the Old Vic Theatre.

[20] He played Simon Lewis in the 2013 film The Mortal Instruments: City of Bones,[21] and followed it with roles in Anita B., The Road Within, Moonwalkers, and The Messenger in 2014 and 2015.

In a 2018 interview with Hot Press, Sheehan described the role as his recent favourite, citing the "magic" and "transcendent experience" of being on stage.

In July, he will appear in Reunion written and directed by Mark O’Rowe for the Galway International Arts Festival.