Sheehan made his acting debut in Aisling Walsh’s acclaimed feature film Song for a Raggy Boy (2003). Since then, he has appeared in several features including the supernatural thriller Ghostwood (2006) and starring alongside Rupert Grint in Cherrybomb (2009). In the last decade, he has appeared in 15 feature films, including the historical fantasy film Season of the Witch (2011) with Nicholas Cage, Ron Perlman and Christopher Lee, Killing Bono (2011) with Ben Barnes and Krsyten Ritter, The Mortal Instruments: City of Bones (2013) with Lena Headey and Aidan Turner, another supernatural thriller The Messenger (2015), and the Moon landing conspiracy comedy Moonwalkers (2015), which stars Ron Perlman and Rupert Grint. In 2018, he starred in three features, including the horror-thriller Bad Samaritan (2018) with David Tenant, the neo-noir sci-fi Mute (2018), also starring Paul Rudd, Justin Theroux, and Alexander Skarsgård, and the post-apocalyptic film Mortal Engines (2018), with Hera Hilmar and Hugo Weaving.
Television includes Foreign Exchange, Young Blades, all three films in the BAFTA award winning Red Riding trilogy, multi IFTA award winning Love/Hate, which he was twice nominated for Best Actor, BAFTA award winning Misfits, which he was also BAFTA nominated for Best Supporting Actor, The Borrowers, Me and Mrs Jones, The Accused, Fortitude, and Genius: Picasso.
On stage he has appeared at the Old Vic Theatre in the title role in The Playboy of the Western World, directed by John Crowley and as Richard III in The Wars of the Roses, directed by Trevor Nunn.