Directors ( or more correctly shareholders) who own or control, directly or indirectly ( e.g spouse or related person owns certain percentage of shares too), 50% or more of the voting rights (usually denoted by their shareholding) in the company. In effect, a shareholder who owns or controls 50% of the voting rights or shares in a company cannot be dismissed and is therefore treated for PRSI classifiscation as a self employed person. The only exceptions to this rule are non executive directors who are also classified as Class S for PRSI purposes in respect of their own director's fees.
Proprietary directors who own or control more than 15% but less than 50% of the voting rights or shares in a company should not be classified as PRSI Class S. Such directors should be classified under the normal PRSI classification rules and will usually be PRSI Class A.
The only significance of the 15% shareholding in a company is that it affects such directors' (or shareholders) rights to the PAYE tax credit and has no bearing on their PRSI classification.
Taken from the IPASS Book 2007