There can never be an 'irrevocable Offer'. From your query, it is clear that you have accepted the offer. Still, unless you join the company and sign the duplicate copy of the Appointment Order as a token of accepting the terms and conditions stated therein, you are not bound by the contract of appointment. Even after that, if you serve the notice period according to the terms of the Appointment Order, you cannot be stopped from leaving the services.
In the first place, why did you accept the offer, and why do you not want to honor your acceptance? Please be clear before giving your acceptance henceforth.
Regards,
Joga Rao