The cost for him to employ you is not the net wage, but the gross one, plus PRSI. It will also involve, additional paperwork to be done, P30's monthly and P35 annually.
The gross wage will depend on what tax credits you allocate to that employment, if not working elsewhere then this shouldn't cause too many problems as all tax credit avaliable, however if your working elsewhere and utilising your tax credits, your income will be taxed at 42% so every €58 he gives you in cash will cost him €100 Plus PRSI.
Its not as simple as this as the cost falls sligtly when you consider the tax saving for him personally.
The revenue may consider if you are doing work for him to receive payment.