McCarthy originated the roles of the Mistress in Evita in 1978 in London's West End, before later returning to the show to play the title role. In between, Siobhan joined the vocal harmony group Wall Street Crash  (1980–1983) where she performed several times at London's Talk of the Town, at the Casino in Monte Carlo, and in two Royal Variety Performances (1980 and 1982).
McCarthy debuted the role of the Mistress in Evita in 1978 at London's West End before reappearing later. Siobhan was part of the vocal harmony group Wall Street Crash (1980–1983) and performed at multiple venues including Talk of the Town, the Casino in Monte Carlo, and in two Royal Variety Performances (1980 and 1982).