The only problem is that the story is very loosely based on Reilly and the events that he was involved in. The sad fact is that nobody really knows exactly who Reilly was and what he was involved with. The most probable thing is that most of the story is fictitious set among a very interesting time in world history.
Yet this does not take one iota away from the magnificent mini-series. You can watch it a number of times and come away with more insight into the turbulent early 20th century. One last note: Leo McKern, was there anything this man did that was not incredibly excellent? His death was certainly a huge loss