Esmé Hamilton lived in Ireland, where all her stories are set, and loved horses all her life. Three of her books at least are about Speedy and her foals, Rainbow, Seafire and Goldfinch.
Finding the books
Most of the books are easy to find: Starlight is easy to find as a US edition, but difficult as the British first.
The Speedy Series
Rainbow and Speedy
Children at Moyinish