Remembered the guy's name: Notker Balbulus (Notker the Stammerer), and Google is a gold mine if you want more. "Notker Balbulus, abbot of St. Gall, d. 975. .... that they played in unison with the voice, or, when they had a harp..."
612: The St. Gall's school of music was founded in Switzerland by Irish harper Cellach, from Bangor, Co. Down, who's name was latinized to Gallus or Gall.
890: Tuathal, an Irish harp and psaltery player, became head master of Europe's leading music school at St. Galls in Switzerland. His name was latinized to Tutilo. Under that name he composed many "tropes" that were published by Pere Schubiger. These tunes contained many characteristics, such as "ground bass" that define Irish music today. "Hodie cantandus" and "Onmipotens Genitor" are two examples.
Legends mentions that the use of harp by the Greeks inspired Irish monks and priests. One of the earliest documented was St. Kevin at Glendalough who is said to have imported a small harp from Greece to accompany prayer chants. His harp had few strings and was probably a lyre.