Mallory makes the point that it is most likely that Ireland's Mesolithic settlers migrated via Britain, and he regards the relationship between the two islands as locked in a long-standing though often less than amicable relationship. He follows the norm for prehistorians in thinking that the Mesolithic inhabitants of both isles as having made little contribution to the genetics of Ireland, but he sees more contribution in the genetic history provided by the Neolithic farmers, who came to the British Isles from the Middle East bearing agriculture with them. It is these who made the great neolithic monuments of the two isles. Significant trade routes emerged and there was a common culture between Britain and Ireland, though in the later Neolithic some cultural divergences arose. Mallory does not analyse cultural divergences in detail, as he is more concerned with origins. However, he discusses the significance of the third group of incomers, the Beaker Folk, a group who introduced metallurgy and possibly brewing to the British Isles.
It is this group who were to give him the kind of bad luck that bedevils historical authors, for he concluded his work only for archaeologists discovered that the Beaker Folk, or a related culture, may have been the beneficiaries of a major pandemic, possibly Yersinia pestis, the Black Death, which swept through the British Isles subsequent to the construction of Stonehenge, possibly killing ninety per cent of the Neolithic inhabitants of Britain and eighty per cent of the Irish population. A hasty amendment had to be made to the book. Unlike the Neolithic folk, who were almost certainly brown, these folk originating in the steppes were white and they were responsible for the white skin of the British and Irish up to our time.
Mallory is luckier in his dealing with the Celtic Invasion, which did not happen. Nineteenth century historians postulated a Celtic invasion of Ireland to account for Celtic cultural elements in Irish art, but later scholars have replaced the invasion with cultural influences in the first century BC. The Irish were not ever a Celtic people, though there were probably cultural affinities with the Celtic tribes of Gall and Spain.
The book deals well with one puzzling element in the relationship between the two islands. Why were there on the East coast of Ireland a few tribes with British names, such as the Brigantes, a tribe from North East England corresponding to the Brigantes of South East Ireland. Mallory raises the issue in the context of political relationships in the British Isles in the pre-Roman period, but the issue is not resolved. He also relates this question to the issue of the languages spoken in Ireland.
One subject that Mallory deals with is the issue of the Romans in Ireland. There was a Roman presence in the isle, but it was not one of conquest, but rather of trade. The author does well to highlight this subject, which has hardly been a topic for popular history. It is known that Rome had cultural impact beyond its boundaries in states outside the empire, so it is not beyond belief that Roman cultural elements occur in Ireland.