Complex sentence: As a member of the Irish American Athletic Club, Robertson competed fro the American athlete, he competed at the 1904 Summer Olympics in St. Louis, at the 1906 Summer Olympics in Athens, and at the 1908 Summer Olympics in London.
Split into simpler sentences: As a member of the Irish American Athletic Club, Robertson competed on the American athlete. He competed at the 1904 Summer Olympics in St. Louis, at the 1906 Summer Olympics in Athens, and at the 1908 Summer Olympics in London.