Commonwealth citizens with grandparents born in Ireland before 1922 (the year of partition) may also apply for UK ancestry visas .
The UK Ancestry visa (UK Patriality visa or 5 Year visa) will open up the entire job market to you for five years and will not carry the restrictions a UK work permit entails.
After five years on the Ancestry visa, you will also have the option to apply for British residency.
Please note, however, that Commonwealth citizens with Irish grandparents may apply for Irish citizenship. This would qualify you immediately for a passport, which would allow you to work anywhere in the EEA. This application would however take in excess of 1 year to process.
It should also be noted that you are responsible for proving your relationship with your Irish-born relatives, and this may require spending time hunting for marriage and birth certificates.