Thomas Cahill's 1995 How the Irish saved Civilization: The Untold Story of Ireland's Heroic Role from the Fall of Rome to the Rise of Medieval Europe was an easy home run for its author, with its appealing premise that Celtic monks preserved the best of Roman-period high culture and literature during the "Dark Ages" following the sack of Rome by Alaric the Visigoth at the beginning of the fifth century AD. Mr. Cahill has a remarkable fluency with the classics, an old-school education that is all too rare these days, combined with a storyteller's ability to tease a world and an epic out of dauntingly scanty and arcane folklore and archeaology. His comparison of the strong and orderly Roman culture abutting wild back-country tribes was compelling.
It is harder to get a grasp of the Pre-Christian Celtic people, but our author is nothing if not into the spirit of the thing. There is often a tendency to "Orientalize" the Irish, but one has to admit that Cahill (who is also obviously fiercely loyal to them) gives us a consistent account of a tough pagan way of life. The relatively quick conversion from warrior culture to monastic society recalls the conversion of Tibet to Buddhism and raises the same kinds of questions.
Cahill continues to impress as a scholar during the extended discussion of the Irish leaders who followed Patrick, about whom he knows a great deal. Of course the old Latin culture continued in the Mediterranean as well, mostly through the vehicle of the Church, but Mr. Cahill is pleasantly persuasive that there was a place under the brush, if you will, off to the side, where some precious endangered shoots of human culture survived for a time. At points there is too much rhetoric around, but part of the difficulty here is filling out a story based on, sometimes, very little.
I also recommend Philip Freeman's The Philosopher and the Druids for some nice imaginative attempts to visualize the ancient Celtic world without taking too much liberty with the known facts. Also there is quite a bit about Patrick including a short autobiography and that is a topic I recommend.