Unfortunately irqbalance has some limitations on some multiple core CPUs. On my Core 2 Duo 6400 CPU it doesn't do anything. I also found this in the man page:

Unfortunately irqbalance has some limitations on some multiple core CPUs. On my Core 2 Duo 6400 CPU it doesn't do anything. I also found this in the man page: This raises a few interesting cases in which the behavior of irqbalance may be non-intuitive. Most notably, cases in which a system has only one cache domain. Nominally these systems are only single cpu environments, but can also be found in multi-core environments in which the cores share an L2 cache. In these situations irqbalance will exit immediately, since there is no work that irqbalance can do which will improve interrupt handling performance.