In December 1983, he won the Boys Under 19 Multiple Event at the Australian All Schools Track and Field Championships in Adelaide with a total of 6708 points.

[1] The following day in Brisbane, he again won the Boys Under 19 Multiple Event at the Australian All Schools Track and Field Championships,[1] this time scoring 7504 points.

Shirley's career personal best score of 8036 points was achieved at the 1988 Olympic Games in Seoul, where he finished 15th behind gold medalist Christian Schenk of East Germany.

His first significant performance as a British athlete was in June 1993 when he won the decathlon at the Azusa Pacific University track meet with a score of 7944 points.

At the 1995 Hypo-Bank combined events track meet in Götzis, Austria, Shirley wore a Superman costume while competing[5] and set a personal best of 14.59m in the shot put.