Here is my usual cold contribution. You claim you were underpaid for years. Yet, you received a playslip every pay day. Surely, you would have noticed the amount you were being paid. Then you discover that you are being underpaid for years and now want arrears paid.
I dont know if you are under contract. I dont know whether you are in private or public sector.
But, in the past few months I hear from a reliable source that some HSE employees who were acting in higher grade posts for years and years were upgraded, but only from a current date. Perhaps this was something involving the Croke Park Agreement. Apparently, there was a vote of workers and the union recommended acceptance and was accepted.
If I were in your shoes I would accept a decent offer even six month arrears. You can fight the company and perhaps even win, but in the long term you could lose.