It looks offhand like you're not being strict enough when you put things back in the IORef, and so it's building up thunks of (+1)...
With two slight mods:
go 0 = return ()
go n = do modifyIORef ior (+1)
go (n-1)
-->
go 0 = return ()
go n = do modifyIORef ior (\ x -> let x' = x+1 in x `seq` x')
go (n-1)
Just a slight prettification of that line:
modifyIORef ior ((1+) $!)
Or applied prefix if you prefer. Prefix ($!) has the nice interpretation as the HOF that makes its argument into a strict function.
Luke