My grandmother, who grew up in western Pennsylvania, pronounced wash and Washington with an intrusive R: “warsh” and “Warshington.” Where does the intrusive R come from in that dialect? It doesn’t seem to be produced by the same mechanism that changes law and order to “lawr and order” in non-rhotic dialects (plus, my grandmother’s dialect was rhotic, if I recall correctly).
According to John Kelly of the Washington Post (Catching the Sounds of the City), he claims:
With the help of Barbara Johnstone, of Carnegie Mellon University, he traces it back to Scotch-Irish immigrants at least a couple hundred years ago.
Midland English is described as "firmly rhotic", where rhotic* (of or pertaining to a dialect of English in which the r is pronounced at the end of a syllable or before a consonant).
Barbara Johnstone, in an interview covered by the article Steel Speak said this:
(There's lots more linguistic trivia in that interview, and I think you'll find it interesting.)
*You may recall my mentioning rhoticity in reference to Why do British Singers Sound American? in response to this question. In that article, the author states that most dialects of England drop their Rs. Go figure, but that's not Scotch-Irish either.
Warsh was used frequently in Quyon and Mayo, West Quebec for at least two generations of Irish Canadians. My father and his siblings spoke what I though was simply bad English, however I now realize that it was an accent transported from Ireland (a la The Great Famine). They also said 'he's an arse', 'Warshington' and 'that'd be a fine how-do-you-do'. Clear, useful and to the point. I hope that dialect is still alive and kicking arse somewhere !