Thanea, Christian daughter to the pagan King Loth, survives being set to sea in an oarless coracle as a druid sacrifice, and is taken in by St. Serf as the only woman in his monastery. In due course, she produces a son who, with his mother, founds the great city of Glasgow. Paperback, 312 pages Published January 1st 1994 by Hodder & Stoughton (first published 1993)

https://www.goodreads.com/book/show/515577.Druid_Sacrifice

Please note: the tag I've listed for time period for this book is the 6th century AD. This is based on wikipedia's estimated birth and death dates for St. Serf (~500 - ~593 AD). I have not, as yet, borrowed this book to verify the information. My apologies if I have provided the wrong time period.