Check Erin's Green Shore in Digitrad. EGS is an English language "aisling" about Daniel O' Connell, but it turns up in American collections with astonishing regularity; so much so in fact that I suspect its popularity there might be attributable to the destruction of the southern American economy afer the civil war.

Seán Báite's summary of the word aisling is pretty spot on. I'm not sure though that the dream vision device was invented to fool the English. Rather, it was an old established convention which was adapted to suit the socio/political climate post Cromwellian invasion. In fact, aislings were traditionally sung in Gaelic and would therefore have been incomprehensible to the occupying forces.

BTW., there is a paper, written by D.K. Wilgus and published I remember not where, but called The Cowboy and the Aisling.