It's a bit harsh to give this only four stars. Generally most things about this book are excellent. The sources are laid out fairly clearly - a bibliography, mostly 20th century and some nineteenth, and 'Miscellaneous Publications' including such things as a BBC programme, one edition of a newspaper, and a lecture. Each chapter has endnotes, and their references match up with the bibliography, at least usually.
However there are some niggles:
 Not many original documents are mentioned, and the presumption is they've been printed accurately. But one can never be sure. To be fair many have probably vanished or decayed or would be difficult to get hold of in the original.
 Reilly often enough says such-and-such a person never visited Ireland, or some similar definite statement; how can he be so sure? No doubt he's likely to be right, but ...
 He doesn't state the official Irish view of Cromwell. We're not all Irish, and some of us haven't been exposed to the Irish education system. Reilly does lay out clearly the object of Cromwell's military expedition, viz to control Ireland, and take lands from Royalists. But it's left rather unclear. Admittedly a revisionist book doesn't have to deal with every aspect of a topic, but the reason Cromwell's of interest in Ireland is exactly because of what he was supposed to have done. (As an example - take 'plantations'. They couldn't have been for spices, sugar cane, tobacco; were they trees? Or what?) Under the rules of the age, was it accepted that a supporter of a losing side should lose possessions?
 He doesn't give details of real or supposed massacres of Protestants before Cromwell got there. (Or subsequent events such as the 'Black and Tans').
 He seems to take Cromwell as a great commander as an established fact. But it certainly appears at first sight as though the main advantage he had was simply lots of cannon of various types. Cromwell just battered away at town walls (and these medieval towns were small - 400 yards was a typical narrowest width). The Drogheda commander seems to have not realised what he was up against.
Some of the reviews here lay stress on one or two documents - and it's often a suspicious sign when conclusions hang on the words of just one or two witnesses, or supposed witnesses. Connoisseurs of this kind of thing will recognise parallels with other atrocity stories, though on a much tinier scale, and parallels with later historians repeating parrot-style. Reilly maintains that much of the force of the 19th century Irish 'rebel' movement was based on fake atrocity stories. The whole idea of Ireland as 'the most distressful country that ever yet was seen' needs a bit of realistic debunking.
I'm sure Tom Reilly started something in 1999, though I wouldn't dare guess how long it will be before he becomes mainstream.