RiGGy and Pete,Your message arrives at an opportune time, as I have just started exploring the connection between Irish and Bluegrass (and Blues) myself. I started playing Irish fiddle tunes about nine months ago. My teacher would occasionally throw in bluegrass like sounds--he would explain that they are not "Irish" or "Celtic" but they sounded so natural, that I was sure the traditions are much closer than the categorization allows.
At any rate, I suggest you also post you inquiry to the Irtrad Listserve. They have a lot of serious students of Irish and Celtic music. If the regulars don't have direct answers for you, they can probably point you in the right direction for your research. I would be interested in what you discover.
The information for the listserve is:
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