Set in Ireland, this tells the story of Ireland when oppressed by the English: a grey stallion appears in the town of Carraroe, but he is actually the shape shifter, Ruairi Mac Eibhir.
R A MacAvoy (Roberta Ann MacAvoy, 1949 - ) is an American author who writes fantasy and science fiction. She was educated at Case Western Reserve University and worked as a computer programmer before she started writing full time. She has written one book which has a horsey element; a fantasy set in Ireland.
Finding the book: in print.
Links and sources
None that I can find - if anyone has any information on the author, please let me know.