I've been playing for many years, but I just started playing in a pipe band. From what I understand, dead drags are a drag interpretation where the grace notes are played as a 'dead' stick (stick pressed directly into head -- very short buzz). To me, it sounds like a flam where the grace note is very tightly buzzed.
As for 'tap rolls', I believe you are referring to roll notation that has an accent over it. These are the typical Scottish buzz rolls that simply begin with a tap/accent.