In order to unlock the Sing the Iron Song Achievement/Trophy you will need to play the Rise of Iron Theme on the bells in the Iron Temple.
The bells are numbered as followed:
Bell 1: The single bell by itself
Bell 2: The 1st bell to the left of the group of bells.
Bell 3: The bell to the right of Bell 2.
Bell 4: The bell to the right of Bell 3.
Bell 5: The bell to the right of Bell 4.
Bell 6: The bell to the right of Bell 5.
This is the order the bells need to be played (the more people the easier it is):
1 2 3 1 5
4 3 2 3 1 3 4
Note: The bells need to be rung in cadence with the actual theme.