Today is the feast of an important, even though not widely known, Celtic saint, St. Adamnan of Iona (also known as St. Adomnan or St. Eunan). He died in 704. Adamnan is generally considered the author of the Life of Columba, his much more famous earlier-generation cousin; the Life is our single most important source of knowledge about life in Scotland and Ireland in the seventh century. He also wrote Of Holy Places, drawing from a variety of sources (especially a Frankish monk, Arculf, who had been there in person) to try to give as accurate a picture of the Holy Land as a Irish monk could possibly give in the seventh century; the work is of particular historical interest in that it has one of the earliest extant maps of Jerusalem (only the Madaba Map is earlier) and the earliest known drawings of major Christian churches (Holy Sepulcher, Apostles on Mount Zion, Ascension, and Jacob's Well).
Adamnan was probably born in modern day County Donegal in Ireland; he somehow received an extraordinarily good education -- nobody knows exactly how -- and eventually became abbot of Iona. He established very good relations with the Northumbrian court and was often called upon to serve diplomatically to assist prisoner exchanges, treaty negotiations, and the like. At some point he had a falling out with his fellow monks (particularly those in Iona's satellite monasteries in Ireland); St. Bede suggests that it was due to his preference for the Roman rather than the Celtic dating of Easter. Whatever may be the case, it's possible that the falling-out was one of his reasons for being so willing to be out and about doing diplomatic work.
In 697, he called and presided at the Synod of Birr; supposedly he had a dream about his mother reprimanding him for not doing more to protect the women and children of Ireland. The Synod passed what has since become known as the Lex Innocentiu, the Cáin Adomnáin, or Canon of Adamnan, in which the priests and kings of Ireland agreed, among other things, to guarantee noncombatant immunity in warfare to women and children. It's unclear how effective it was, historically, and it likely had no enforcement beyond the honor system, but the agreement in itself was a diplomatic achievement, and a forerunner of later attempts to reduce the brutality of war by legal means.