Is the song ring around the rosie a song about hiroshima?

No.

Ring Around the Rosie pre-dates America's nuclear attack on Hiroshima by at least 60 years.

The first recorded version of Ring A Ring Of Roses dates to 1881, when it appeared in Kate Greenaway's edition of Mother Goose:

Ring-a-ring-a-roses,

A pocket full of posies;

Hush! hush! hush! hush!

We're all tumbled down.

It was however, referred to twenty six years prior to that in Ann S Stephen's novel The Old Homestead, which describes children playing 'Ring Ring A Rosy' in New York.

In 1883 William Newell reported two versions in America, and claimed that one version was current in New Bedford, Massachusetts in 1790:

Ring a ring a Rosie,

A bottle full of posie,

All the girls in our town

Ring for little Josie.

Also in 1883, versions were recorded in England which included the now familiar sneezing motif, for example:

A ring, a ring o' roses,

A pocket full o'posies-

Atch chew! atch chew!

In 1892 Alice Gomme listed twelve versions, including one like the version currently sung in Britain:

Ring a-ring o' roses,

A pocketful of posies.

a-tishoo!, a-tishoo!.

We all fall down.

After World War II, historians began to claim that there was a connection between the rhyme and the outbreak of Bubonic Plague in 1665, or possibly even the outbreak of the 1300s. However, these claims are generally regarded to be incorrect because of the lateness of this explanation arising, the fact that the symptoms of plague do not actually match the Words of the song, and that earlier and foreign language variations of the song do not match up to the theory.

There are however, many people who still subscribe to this theory, despite the fact that it is highly improbable that it's correct. For more, please use the link below.