I bought this book in Dublin in 2004 during a business trip because I was intrigued by this claim. I read it and liked it, despite finding this central claim rather strange and the "evidence" on which it was supposed to be built rather shaky. I picked up again, quite recently and have re-read it. I have enjoyed about just as much as the first time, despite still finding it just as speculative and unconvincing. The central claim of this book is that, contrary to conventional history, the Romans invaded at least once (and maybe twice) and occupied at least part of Ireland and that this can be "proved" in a number of ways.
Before going any further, I should make two quick comments. One is that this is yet another piece of "revisionist" history where an author tries to demonstrate that what we have been taught to believe is not "the truth", which has been somehow deformed, twisted and hidden as the result of some sort of "hidden agenda". It is becoming increasingly common to come across history books claiming to tell the "true story" unlike all of their predecessors on the same subject. Some of this may be enjoyable and come up with a plausible, if speculative, story. Other are simply too far-fetched to believe.
In this case, whether - and perhaps more importantly to what extent - you believe the book's central claim is entirely up to you, although there is also a need to recognize that this kind of feature is also a marketing ploy. This brings me to the second point. Rather than opposing the traditional claim that Romans never set foot in Ireland and did not exert any influence over it to the claim that Ireland was invaded and partly occupied, it may be more likely that something in between happened. This is one of the reasons why this book has value, even if the author stretches whatever "evidence" he has too much.
Ireland must have been subject to some Roman influence, even if only through trade, piracy and retaliatory raids to destroy the pirates' nests and restore "Roman order". This would be enough to explain most of the so-called "evidence" of Roman influence in Ireland, starting with Roman coins. It might also explain the presence of a fort that the author claims to be a Roman one. If this was so, it could simply be a temporary outpost set up to suppress piracy, although there may be a range of other interpretations as well, such as an Irish chieftain, client of the Romans, whose stronghold may have been built with Roman techniques.
Then, as another reviewer mentioned, there is the author's work in demonstrating the Roman influence on many aspects of Irish culture and society. Here again, some of the points made may seem far-fetched, speculative, or simply exaggerated, and maybe they are. However, such questioning does has merit. They show that the assumption that Ireland remained entirely immune from Roman influence is rather implausible, to put it mildly. It would be particularly difficult to believe since all areas just beyond The Empire's frontiers, but also areas as far away as Scandinavia or Poland did feel Rome's influence. Such a unique exception would beg an explanation.
This is why I found this book was worth four stars or anything between three and four because, to be truthful here, I took a while to make up my mind on the rating. While the author's claims are often far-fetched or exaggerated, and although he does jump to conclusions and infer rather a lot from very limited evidence, he does show how implausible it is to believe that Ireland was not influenced by Rome. The unresolved question is how much influence?