Read the following song and answer the question.

In Dublin is fair city,

where the girls are so pretty

I first set my eyes on sweet Molly Malone

As she wheeled her wheel-barrow

Through streets broad and narrow

Crying cockles and mussels, alive, alive-O!

Alive, alive-O! alive, alive-O!

Crying cockles and mussels, alive, alive-0!

She was a fish-monger, but sure twas no wonder

For so were her father and mother before

And they each wheeled their barrow

Through streets broad and narrow

Crying cockles and mussels, alive, alive-O!

Alive, alive-O! alive, alive-O!

Crying cockles and mussels, alive, alive-O!

She died of a fever, and no one could save her

And that was the end of sweet Molly Malone

But her ghost wheels her barrow

Through streets broad and narrow

Crying cockles and mussels, alive, alive-O!

Alive, alive-0! alive, alive-0!

Crying cockles and hussels, alive, alive-0!

What is the song about?