If you occasionally do short-handed (1 or 2 people) racing, and your boat is in a different configuration for this, you can apply for a separate Short-Handed certificate, once you have a 'primary' certificate for your crewed configuration. The fact of only having 1 or 2 crew does not affect the rating, since IRC does not use crew weight in the calculation; the rating will only vary due to differences in sail data or configuration.
Note that if you only ever race IRC in the short-handed configuration then there is no need to hold the extra certificate.
IRC Rule 8.2.1. says:
"A boat may additionally hold a separate short-handed certificate. This short-handed certificate shall be valid only for racing in classes, or divisions of classes, for no more than 2 crew, included in a Notice of Race. The short-handed certificate will be clearly identified and shall only vary from the primary certificate in respect of, mainsail widths, headsail dimensions, single furling headsail allowance, the use of stored power, SPA, STL, spinnaker pole/bowsprit, moveable ballast and variable ballast. A boat holding a shorthanded certificate may use only that certificate for races for no more than 2 crew."
The application form to apply for a short handed IRC certificate can be obtained from your Rule Authority.