Garret Fitzgerald was aware of the offer and it's details, as were the ICJP, as such the British would have found it very difficult to renege. This is covered in the following extract from his autobigraphy.
Solution Sabotaged – Garret Fitzgerald (1991) - July 1981
Nit picking as always. I have stated that I don't know what the Kitchen Cabinet's motivations were when they made the tactical error of rejecting the British offer. They have yet to offer any explanation so people will draw their own conclusions. I believe that it is possible that Dixie Elliot is right but without a response from those who took the decisions there is no way to know.