The Aran Islands by J.M. Synge (1907) — Synge spent several summers on the Aran Islands at the end of the nineteenth century at the suggestion of W.B. Yeats. His account of life on Inishmaan is one of the great pieces of Irish travel writing, and it captures a world that had more in common with the medieval past than with modern Europe. Reading it before taking the ferry from Doolin gives the crossing an extra dimension.
The Burren: A Companion to the Wildflowers of an Irish Limestone Wilderness by E. Charles Nelson and Wendy F. Walsh — The standard botanical guide to the Burren, with illustrations and descriptions of the rare and remarkable plants that grow on the limestone pavement. Useful if you are planning to walk seriously in the Burren rather than just cross it.
Tim Robinson's Connemara trilogy — Robinson spent decades mapping the west of Ireland on foot and his accounts of the limestone landscapes west of Galway illuminate the whole Atlantic edge of Ireland, including the Clare coast. Start with Connemara: Listening to the Wind.