I haven't checked the detail of the employers PRSI. I think if t/o is below a certain level the 10.75% may not apply (in full, at any rate).
But, and I'm subject to correction on this, I don't think the fact that it's director owned makes any difference.
I'd double check the PRSI thing if I were you. The last thing you want is a retro claim in about 3 years time, plus interest, plus penalties etc.
On the more general point, there is a rough rule of thumb about incorporation which is sometimes used. It's basically that when ones income exceeds what you would expect to earn as an employee maybe it's time to incorporate, or examine it anyway.