Complex sentence: The event disillusioned McCullough with the IRB, and he soon took it upon himself to revitalize the organization, which over the years he did with the aid of Bulmer Hobson and Sean MacDermott.
Split into simpler sentences: The event disillusioned McCullough with the Brotherhood, and he soon took it upon himself to revitalize the organization. He did so over the years with the aid of Bulmer Hobson and Sean MacDermott.