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.