Title: Druid Sacrifice
Description: 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.
Subjects: Pagan Sacrifice, Monastery, Historic, Glasgow, St. Serf
Places: Glasgow, Scotland
People: St. Serf, King Loth, Thanea (daughter to King Loth), Pagans, Christians, Druids
Time periods: 6th Century, Dark Ages