If you join the company scheme, then I don't think you can continue to claim tax relief on your contributions to your existing PRSA.
As Liam mentions above, you'd need to take out an "AVC PRSA". The difference is from a tax perspective. You can claim tax relief on contributions to the AVC PRSA. But if you leave your current employment, then you should stop contributing to the AVC PRSA as it's linked to the occupational scheme of your now previous employer.
TBH, I think it's too much hassle. I would just cease paying into your current PRSA and put all contributions through the company scheme. Most likely the company scheme charges are lower than the PRSA charges. And you don't have to do everything through payroll. You can, say, choose to make 10% AVCs through payroll (where you get tax relief at source) and you can make additional AVCs directly to the pension company, and claim tax back at the end of the year. I believe nearly all pension funds allow you to make such irregular AVC contributions. But check it with them just in case.