I met some young Irish fellers from a small rural town (forget which) a few years ago... they were working their way across the U.S. They started out in NYC, yet despite the huge Irish population there, they had some trouble understanding the local urban speech (when I met them we had a little difficulty, too, but after agreeing to slow down and share a few pints all was well
).
They wrote me later to tell me that when they hit the Appalachian mountains (a relatively poor section of the Eastern US, much of which was settled by the Irish) they found they could understand almost all and were amazed at the expressions they heard that were current both in that rural area and in their rural hometown. There are many pockets in Appalachia even today that are geographically/economically isolated linguistic communities, with expression that date back to the Elizabethan era (no, I can't think of a single example right now). You'll also notice the similarity between bluegrass and Irish music.
