You should clarify with the liquidator the precise nature of your role. Is it as an employee? Is it on a self-employed basis? I would assume that it was as an employee, but you never know until you confirm this. And of course you will have to tell social welfare about this role, it may be that you are still employed and not entitled to a Jobseekers payment at all!!!!
As far as your RP50 is concerned, this is for redundancy, not for backpay. There is a separate claim for claiming backdated pay.
If the P45 was not accurate, and you have relied on this for making a social welfare claim, then you have not acted correct, as you were making a claim for a period when you were employed (even though you did not receive wages).
Social Welfare don't usually backdate claims for Jobseekers before the date of the claim (although they have the power to do this). Have you already got a decision on your claim? If so, then they will consider this an overpayment and ask for it back. If you go to them with teh information they are less likely to consider that you have committed a fraud, although this is also possible. If you haven't received a payment already you should let them know about your mistake.
Your action for those two weeks is of course against the employer for not paying you wages, not a social welfare claim. If your employer can't pay you, then you can make a claim to the Social Insurance Fund, the liquidator should keep you right.
What you probably should have done was to speak to your Community Welfare Officer about a payment for the weeks where you weren't getting paid, although its too late to do that now.