Esentially, the two main chords are C and G.
If you know the song, then the order should be fairly obvious. 
 
I belive the first line goes: 
C                                         G
Rosie had a boyfriend, his name was Tiny Tim, 
                                                C
She threw him in the river, to see if he could swim, 
                                               G
He swam across the rivers, he swam across the falls, 
 
Rosie got excited, and she grabbed him by the... 
 
(The word balls is never said, but should launch into the Chorus) 
C
...Bang, bang Rosie, Rosie bangs all day, 
G
Who's gonna bang for Rosie, when Rosie goes away? 
 
It varies, depending on which part of Ireland you're from, I think. 
 
So have fun with that.