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: 
C5                                                                   G5 
Rosie had a boyfriend, his name was Tiny Tim , 
                                                                        C5 
She threw him in the river, to see if he could swim , 
                                                                               G5 
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) 
C5 
...Bang, bang Rosie, Rosie bangs all day, 
G5 
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.