He studied composition with Jane O'Leary, Eric Sweeney and Seóirse Bodley.
He also attended masterclasses by Messiaen, Iannis Xenakis, Pierre Boulez, Luciano Berio, and Helmut Lachenmann in France.
Holohan has composed for solo instrument, ensemble, orchestra, stage, choir and voice.
He has also collaborated with a number of poets including Nobel prize-winners Seamus Heaney and Tomas Tranströmer, Ivan Lalic and Paul Durcan.
Career highlights in Drogheda include the performances of Cromwell at the "Drogheda 800" celebrations (RTECO, Lourdes Church, 1994); The Mass of Fire, Augustinian 700 anniversary (RTÉ TV live broadcast, 1995); No Sanctuary with Nobel Laureate and poet Seamus Heaney (Augustinian Church, 1997); Remembrance Sunday Service, at Drogheda Unification 600 (RTE TV live broadcast, St Peter's Church of Ireland) and two major concerts with The Boyne Valley Chamber Orchestra at Fleadh Cheoil na hÉireann in 2018 and 2019.