The following is a link to the Ring a Ring Of Roses FAQ by a chap called Ian Monro:
http://www.ualberta.ca/~imunro/ring.html
He
says 'First, the rhyme's not old enough. Second, the early versions are clearly not about the plague.
The earliest printed source for the rhyme dates from 1881. A folklore book published in 1883 claims that versions of the rhyme were circulating in Massachusetts in 1790, but no printed evidence is available.
This earlier date is 125 years after the last major plague of the English-speaking world, and roughly 450 years after the Black Death, the 14th century plague most commonly associated with the poem.
Furthermore, most early versions of the rhyme would be extremely difficult to interpret as references to the plague. Folklorists have come up with far more plausible explanations for the origins of the rhyme'.
I would advise you check out his page for a far more detailled answer from Mr Monro himself, rather than me take the credit for his hard work.