Bass does not come in until the heavy bit (1:34) then comes in with main riff:

G[----------------------------------------------------]
D[----------------------------------------------------]
A[5----5-5-3-5-----5-5-3-1-----1-1-0-1-------777777---] Main Riff
D[-0000--------0000--------0000--------000000---------]


Then at about (2:05) the second riff kicks in:

G[------------------------------------------------------]
D[------------------------------------------------------]
A[-5555555555555555558\-----111111111111111111111111----]
D[------------------------------------------------------]

then at about (2:36) go back to main riff until the song reverts to 'acoustic mode'

then last time you play the main riff it changes slightly, as far as i can make out it 
something
like:

G[----------------------------------------------------]
D[----------------------------------------------------]
A[5----5-5-3-5-----5-5-3-1-----1-1-0-1-----------01~--] Main Riff
D[-0000--------0000--------0000--------0000000023-----]

hold on the last note.

the bass in riff 2 is hard to make out, i hope i've got it right, if not then please 
me.

alternate second riff. Sometimes i play this, it sounds good also:

G[------------------------------]
D[------------------------------]
A[-555555-333333-555555-888888--]
D[------------------------------]
                                     >Second riff
G[------------------------------]
D[------------------------------]
A[-111111-000000-111111-333333--]
D[------------------------------]

 (forr alternate riff 2: there is no gaps in time between notes, i put gaps on the tab 
make it easier
to make out the notes)