[1] Erna began his professional career in the Boston garage band Jared, which produced its single "Space Traveler" in 1980.
In 1996, Erna, The Outlets' guitarist Rick Barton, singer Mike McColgan, and bassist Ken Casey formed the Dropkick Murphys.
Erna would appear on the band's earliest 7" records, compilation appearances and their Boys on the Docks EP before leaving the band in 1997 shortly before the recording of their debut album, Do or Die.
[2] In 2002, Erna along with Mike McColgan formed the punk band Street Dogs.
Erna has also been the drummer for a reformed Nervous Eaters, one of Boston's first punk/new wave bands.