The bows from waterford are so obviously viking its not even coincidentall, some of them look pretty much like hedaby bows,nocks and all, other have ornate type of pin nock,but once again thing not unknown in germanic environ. Waterford was raided by vikings like 4 or 5 times and burned down couple of times. I wouldnt be surprised if numbers of fighting tools were lying around just uncovered. What we do not seem to perceive, that generic longbow is so simple thing, that saxon would not consider viking bow any other type of bow than his own and that later medieval english archery is not some homogenous body which appeared at once from mysterious source (like my favorite "english longbow was brought from wales"), but rather synthesis of similar archery traditions of different tribes, which eventually destiled along socio-cultural contact.
We have for example saxon bows and viking bows, both from people who came into england and settled there and apparently there was indigenous british archery tradition before and as usually this all mixed.
The fact that the level of technology among them was similar and that they in more than one case shared congeniall language made all this much easier.
J.