In 1967, McCullough moved to Belfast where he joined Chris Stewart (bass), Ernie Graham (vocals) and Dave Lutton (drums) to form the psychedelic band the People.

Later that year the band moved to London and were signed by Chas Chandler's management team, who changed the group's name to Éire Apparent.

Under Chandler's guidance after a single release they toured with groups such as Pink Floyd, Soft Machine, the Move and the Jimi Hendrix Experience, as well as Eric Burdon and the Animals.

[10] He spent more than a year in the band, playing lead guitar on several singles, including "Hi, Hi, Hi", "Live and Let Die" and "My Love", as well as on the album Red Rose Speedway.

[11] McCullough's spoken words "I don't know; I was really drunk at the time" can be heard on the Pink Floyd album The Dark Side of the Moon (1973), at the end of the song "Money".

[12] In 1975 McCullough joined the Frankie Miller Band with bassist Chris Stewart, keyboard player Mick Weaver and drummer Stu Perry.

[7] McCullough played concerts as a session musician with Roy Harper, Frankie Miller, Eric Burdon, Marianne Faithfull, Ronnie Lane and Donovan.

McCullough performed at concerts in Northern Ireland and Scotland, playing with a backing band (featuring Stephen Quinn on drums and Sean McCarron on saxophone).

In the same year, McCullough started to work with Dave Sharp from The Alarm) and together they enlisted keyboard player Zoot Money, bassist Gary Fletcher and drummer Colin Allen, a line up which became known as the Hard Travelers.

[15] McCullough remained active in the European music scene and played regular live gigs with artists including Ed Deane, James Delaney, Noel Bridgeman, also John Quearney.

[20] The backing band was named Henry's Heroes, and included Tim Hinkley, Mel Collins, Neil Hubbard and John Halsey[21] and bass player Kuma Harada.

[22] On 14 June 2016, his wife Josie confirmed that McCullough had died at his home at Ballywindelland, Ballymoney, County Antrim, earlier that morning after a long illness.