The birq-1.5.1 is bugfix release. The command line options "--threshold" and "--load-limit" were fixed (same options in config file were ok). The 1.5 branch has a mess with command line options, config file options and config re-read by SIGHUP. So it's recommended to use new birq-1.6.0.
The birq-1.6.0 has new config file option "use-cpus". It's a affinity mask. It can coexist with older "exclude-cpus" option. The real affinity will be 'affinity = <use-cpus> & ~<exclude_cpus>'. The command line options for IRQ processing (like strategy, threshold, intervals etc.) will be removed. That options can be defined by the config file only now.