If your Occupational Scheme is not a PRSA then you can ignore the Employer contribution in determining the limit to your contribution. So is you are paying 15% currently into the Occupational Scheme, then you can pay a further 15% (total 30% based on age) as an AVC, either into the Occupational Scheme or into a stand-alone PRSA AVC Plan.
If your main scheme is a PRSA, then the overall limit of 30% applies (i.e. Employer + Employee contribution).