From medieval times onwards there were close connections between the Gaelic and other European cultures.
Gaelic scholars were learned in the Christian writings and also in ancient culture - classical texts were translated into Irish. It seems likely that Gaelic music theory was based on classical theory as transmitted by Boethius.
The early history of the harp in Europe is obscure but there seem to have been harps played in Scotland and Ireland from early times.
In later centuries we have more concrete evidence of Gaelic harpers working abroad, especially in the 17th century when they were employed in the Royal courts in London, Denmark, Spain, and Poland for example.