His father, a native of Glin, County Limerick, was a member of the Garda Síochána (the Irish Police Force) stationed in Dublin.

Enthusiastically supported and encouraged by his parents, he attended the Municipal School of Music (now known as the Dublin Institute of Technology) where he studied classical violin for more than five years.

[2][4] His interest in American bluegrass fiddle music[2] also influenced his style, as can be heard in tunes like "Flop Eared Mule" (also called "Donkey Reel"), recorded with The Dubliners in 1968,[5] 1969[6] and 1983.

[2] After 50 years of playing and after the death of founding member Barney McKenna, in the autumn of 2012, Sheahan announced the retirement of The Dubliners by the end of the 50th-anniversary tour.

[13] Sheahan said he was working on a solo album which would contain a collection of unrecorded compositions he had written over the past 50 years and was developing a book containing his poetry.

In September 2014, he performed with the RTÉ Concert Orchestra conducted by Gearoid Grant, live in Meeting House Square for Culture Night 2014.

The tune was subsequently covered by a number of other artists and was followed by the release of an album of original instrumental pieces, written and recorded together with classical guitarist Michael Howard.

[23] Another album, entirely of Sheahan's compositions, was released in 2008 with the title "The Marino Suite" and features the Young European Strings Chamber Orchestra[24] as a supporting ensemble to his fiddle playing.