The (Irish) monks at St. Gall monastery in Switzerland not only played harp in medieval times, but their instruments were world-renowned. I remember reading an old account of this in the British Museum Library but the details now unfortunately escape me. From:

http://www.libraryireland.com/IrishMusic/II.php

it says

All musical persons have read of the world-renowned monastery of St. Gall, in Switzerland, but the fact is too often ignored that its foundation, in the year 612, was the work of the Irish saint Cellach, whose name has been latinized Gallus or Gall. This great Irishman, a student of Bangor, Co. Down, the friend and disciple of St. Columbanus, died October 16th, 646, and, at his demise, the fame of his music-school became known far and near.

Another early reference to the Irish Harp is in a distich on the death of St. Columba (d. 596), wherein we read of a "song of the Cruit without a ceis," that is, "a harp-melody without a harp-fastener [ceis]," or an air played on an untuned harp. Regarding our Irish cruit Sir Frederick Gore Ouseley, Bart., Mus. Doc., says:--"From its very construction we must assume that Harmony was known to the ancient Irish.

If you Google "st gall monastery harp" it brings up a bunch of other links, one of which is a Google-books page (not pastable) from the Ulster Journal of Archaeology which says

Performance on stringed instruments, and especially on the harp was in fact ... distinctly mentioned as such among the learned men of St. Gall...

There's more in that vein, but it conveys the idea. About the abbey itself you can check out good old Wiki:

http://en.wikipedia.org/wiki/Abbey_of_St._Gall