The accountants charge 3% of your entire income to cover all their fees. This was the deal when I saw the contract 2 years ago.
Aside from this there are several other costs. However as I'm not familiar with the Irish tax system, NHS, insurance etc I can't say how big of an impact this will have on your income.
These CXC and Mcnamarra schemes are the direct result of an investigation by the Irish treasury into Dublin. One fine day someone woke up and realised that year after year Ryanair had been increasing the amount of aircraft based in Dublin but magically the amount of Ryanair employees had stabilsed.
Eventually the treasury declared the Brookfield contracts as they were insatisfactory and this is where the Irish ltd. companies come in. They basically serve as a tax buffer between the Irish government and Ryanair. It doesn't cost Ryanair anything, but the Irish get their money regardless. Everybody happy. Except for the poor kid who gets squeezed for every euro he earned.
Working with contracts is much cheaper for Ryanair. What little in extra pay they have to give you is offset by not having any administration costs, social security contribution, no required insurance coverage, no pension schemes. Etc...
But for the most part it is administration. Remember that even though we fly nearly 300 aircraft, the White House is staffed by just a handful of people. Administration costs have to be kept down for Ryanair at all cost.