What was Phil Lynott's most successful group that included Brian Downey in four songs?
Back on the Streets (Gary Moore album):  Thin Lizzy bassist/vocalist Phil Lynott and drummer Brian Downey appear on four songs, including "Don't Believe A Word" (which originally appeared on the 1976 Thin Lizzy album "Johnny the Fox") and the UK top 10 single "Parisienne Walkways". Phil Lynott:  His most commercially successful group was Thin Lizzy, of which he was a founding member, the principal songwriter, lead vocalist and bassist.
Thin Lizzy