The Instrument Rating is a requirement for a CPL to fly IFR. Note that you can have a CPL without an instument rating but the commercial flying opportunities are somewhat limited. There are a fair number doing parachute drops, crop dusting, flying tourists around game reserves etc. If you want an airline job you are going to need an instument rating.
The IRR is a simplified rating which replaces the old uk-only IMC rating. It is designed to improve PPL safety by giving the skills required to safely land in IMC. It does not give you the right to commence a flight in IMC conditions. Highly recommended for any PPL as insurance against changing weather and to get above the clouds.
So it depends on your objectives - if you want an airline job get the (more expensive) IR, if you want to stay safer as a PPL get the IRR.