I recognized the author as the editor of one of the editions of Calvin's "Institutes of the Christian Religion," and was surprised to learn that he also wrote a book on the history of the Celtic churches. Apparently this was a particular interest of his, and he applied his historical scholarship to giving us a well-researched book. As he says at the end of his preface, his book is a "book of generalization that takes account of the accumulating body of discussion," meaning that there are many times where he compares so-and-so's view to another's view to the current consensus which might be right, and in any case this or that was the case. In other words, the reader does learn what historical facts are the most reliable ones concerning the Celtic churches, but it is nowhere near as exciting or inspiring as reading Bede's "Ecclesiastical History of the English People" written in the 8th century (this might also be because McNeill categorically discounts miracles and supernatural events). I rate it somewhere between three and four stars.
"With respect to asceticism, the gulf between Celtic Christian and Protestantism is very wide. Nineteenth-century Protestants sometimes tried to identify the Celtic church polity with their own. A kinship might be claimed far more plausibly on the basis of the acceptance of scriptural authority. The Bible was read with orthodox presuppositions but with personal directness...The message of Celtic Christianity was borne through many lands by an army of heroic, ascetic adventures, in dispersed bands whose only head was Christ, implanting a culture in which a primary reverence for Holy Scripture was harmoniously matched with a delight in classical literature." (p. 192)
"The austere and unrelaxed devotion of the Celtic saints is so alien to the modern spirit as to be now almost incomprehensible. We are not familiar with men who choose to subsist on one meal a day, who sleep on hard floors with their heads on pillows of stone, who spend many hours of the day and night singing psalms and hymns, and yet with unfailing energy exercise creative leadership, changing the society in which they move and schooling barbarous peoples." (p. 223-224)