The Irish and the Highland Harps by Robert Bruce Armstrong is an exceptional document published at the beginning of the 20th century. Now available online, it is also downloadable from the archive.org website. The numerous illustrations and plates of early harps reinserted in their historical context such as the 1792 Belfast Harp Festival followed by the work of Bunting, completed with some arrangements of music by the author himself give both musical and historical perspectives to this book.
ARMSTRONG (Robert Bruce), The Irish and the Highland Harps, Dublin: Irish University Press, 1969 (1st edition: Edinburgh: David Douglas, 1904).
Robert Bruce Armstrong's second volume evoking hybrid instruments, English and Irish Instruments is online here.