Traditional Christian prayers handed down through the ancient scribes and saints, and revealing insights into the rich and varied theology of the early Church. There's a lot to be said from studying early prayers such as these as they provide a continuous link of worship between the early Christians of the first few centuries after the Crucifixion to ourselves. Their content and pattern provide a useful template for the writing of modern prayers.
Traditional Celtic blessings and prayers
Carmina Galetica - traditional Scottish prayers
The Stowe Missal (a very early liturgy considered to be Celtic in origin and pre 600AD). This is a pdf of a translation found on the web and assumed to be freely available.