In these “pre-NCT” checks, you could very well end up paying them to check or fix something that might necessarily be a problem, while they could miss out on something that could fail in the test. While you will probably not get a guarantee that your pre-checked car will pass the NCT, the garage you use may show goodwill by fixing whatever it is causes your car to fail. But they’re not obliged to, so you could be unlucky.
You should save yourself some money and just get your test done – doing all the easy stuff to make it more likely that you’ll pass the test first time.
If during the test anything causes the car to fail, you can specifically get that fixed by your local garage and get a re-test done for €27.50. Make sure you get a written quote for the cost of labour and materials required to fix the problem before you get any work done as well. In the long run, this will all probably save you money, rather than getting the “pre-NCT” done up front.