The first novel published by the famous writer priest, which takes the Grail legend and recasts it in the setting of late pagan / early Christian Ireland; it was an early entry on my (now out of date) list. The love between a mature king and a young slave-girl (yeah, really) is a central plot strand. I've enjoyed several of Greeley's later books, but I'm afraid this one was rather boring when it wasn't being skeevy, and the odd spelling variants in Irish names and words grated as well ("Podraig", "Tanaise", "Agadhoe"). I was musing the other day (in a piece that I hope will appear in the Eastercon programme book) how few recent writers are able to take traditional Irish/Celtic themes and do something new and convincing with them, and I'm afraid this book is further testimony to the difficulty of that task. You can skip it in good conscience.
This came to the top of my reading pile as the sf/fantasy book that I had acquired longest ago (in 2009) and not yet read. Next on that list is A Princess of Roumania, by Paul Park.