O’Neill wrote several Undertones album tracks and singles, usually writing with bassist Michael Bradley.
In 1983, Damian formed That Petrol Emotion with his other guitarist brother, John.
Since The Undertones reformation in 1999, O’Neill continues to perform and record with them.
Since 2012, Damian O’Neill and three of his former Petrol Emotion band-mates, Raymond Gorman, Ciaran McLaughlin and Brendan Kelly, have been performing as London-based The Everlasting Yeah, playing kosmische Krautrock influenced Musik mixed with most individual soundscapes creating sheer amazing atmospheres with powerful guitary, percussionized tunes. Their first LP Anima Rising was released in late 2014.
In 2001, O’Neill released the experimental electronic album A Quiet Revolution on the Poptones label, while as X-Valdez he released a 12 “Higher Grace” with arrangements by Xavier Jamaux and Athena Constantine, and appeared on Toy’s Factory in 2000.
In November 2014, he released a solo-single “Trapped in a Cage” on 7″ limited edition vinyl.