You have to look beyond the money here.
1) You should make sure that you do not do anything illegal or improper.
2) In some professions, you would be required to report wrongdoing, but I presume that it does not apply here.
3) If you are under pressure, you should leave.
Any form of blackmail is reprehensible. You should just leave if that is what you want to do.
You could ask for a termination payment on the grounds that you are being constructively dismissed by being asked to do illegal things which makes your position impossible. But you should not couple that with "And if you don't give it to me, I will go to Revenue..."
I think that the best solution is to tell them that you can no longer work there and so you are giving 6 months' notice. You want to be paid for the 6 months but you are going to look for another job. If you get that, then you won't need any further compensation.
If you walk out without another job to go to, it will be difficult to explain that to a potential employer.
Brendan