There is no doubt about it, this is a deeply flawed book. It is, however, an enjoyable read, if you are prepared not to let those flaws annoy you. The story of how classical texts were preserved in Irish monasteries isn't well enough known, and it is true that, once the enlightened minds of the Renaissance started looking for them, many of the surviving texts were traced in monasteries with strong Irish connections. And the tale of St Patrick, familiar to every Irish schoolchild, is illuminating and interesting enough to be worth telling to a new audience.
Unfortunately Cahill, who is clearly writing for a US audience, combines a strong personal agenda with a lack of historical knowledge. He repeatedly characterises early Irish christianity as "Catholic" in the sense of "as opposed to Protestant" when not only is that anachronistic, but Christianity in early Ireland wasn't even centred on Rome (it was inspired by the Desert Fathers of North Africa; differences were only resolved at the Synod of Whitby in 664). He uses arguments based on the shape of the early church in the post-Roman provinces when these are irrelevant to Ireland's totally different tradition. He is equally weak on Irish pre-Christian culture, unable to decide whether it survived into the Christian era or was wiped out by Patrick.
Cahill's bias lets him down repeatedly, and he often wants to have his cake and eat it. Early on he cites as an example of vile anti-Catholic propaganda the tale that, in Irish parochial schools, the 'nuns told their charges never to order ravioli on a date lest their boyfriends be reminded of pillows'. I'm sure anyone who was schooled by Irish nuns in the 50s or 60s could top that with even more hilarious examples. Calling traditional Irish Catholicism narrow-minded and sex-obsessed is not a libel, but Cahill has too much of an axe to grind to see it.
More seriously, Cahill also ignores the role of Byzantium and the Eastern Empire - which only fell in the 15th century, AFTER the start of the Renaissance - and of Arab scholarship; at least, if not more, important in preserving classical texts. Claiming that, without the Irish monks, the knowledge of the classical world would have vanished without a trace is overstating the case.
By presenting his arguments so poorly - in many places the book is incoherent, his arguments contradictory - we find ourselves doubting whether any of this is worth paying attention to. There is a place for a good, popular book decribing the importance of the north, Ireland included, in early Christianity and the preservation of scholarship. If you are seriously interested in the way the late Roman Empire turned, Transformer-like, into the Roman Church, try The End of Antiquity: Archaeology, Society and Religion in Early Medieval Western Europe
or The Inheritance of Rome: A History of Europe from 400 to 1000
, though I wouldn't describe either as a light read. Sir Kenneth Clark's Civilisation
also has a good section on the role of the Celtic West in preserving Christian culture; shorter, far more readable and with better pictures.