I have British and Irish passports. Is it possible for me to get a 5 year year visa ?

Yes, according to the China visa policy, UK citizens, but just eligible persons, can obtain 5 year multiple entry visa. Usually, if required by the applicant, Chinese authorities will offer 2 year multiple entry visa to "usual" UK citizen. To be eligible person, at least as far as I know, you have to have previous clean record on traveling relatively often to China on shorter term visas. So, I honestly doubt that you will be considered eligible person if you just plan to visit China for the first or second time. Irish passport is not eligible for a long term Chinese visa.

Thank you. I lived in China for 22 years up to 2013 and have made 3 short term visits since then. Hopefully that will help with 5 year travel visa.

Of course. These detail that you have mentioned, definitely makes you an eligible person.