You can however have your workers on:
- contract for specified work with clause of the transfer of author's economic rights (umowa o dzieło z przeniesieniem praw autorskich) - the whole pay is under 50% tax rebate.
But be aware that these contracts shouldn't be used if the work is permanent - it's a minefield, and not a good idea to blindly issue such contracts if you aren't fully aware of the consequences.
- contract for specified work with a self-employed one-man-business - which is possibly the best for you; there is no tax rebate but on the other hand many costs could be deduced by the business owner, moreover new startups has 2-years vacation with 70% reduced SS overhead.
You won't employ many people on this basis - unless there's something in it for you. Full time staff are very unlikely to be swayed, unless the 800zl+ cost of social insurance is cheaper than paying it through a permanent contract. It's also got a very bad reputation in Poland - educated, decent staff are unlikely to agree. You should also be very aware that the tax office does have the power to declare this sort of agreement as invalid - and the fines usually amount 100% of unpaid (in their eyes) social insurance contributions, plus of course the contributions must be repaid too.
My recommendation is to compare the costs of hiring people in Ireland vs Poland on normal work contracts. If it works for you, then you can investigate ways of saving money - but don't assume that you can get away with hiring people on contracts of dubious legality.