Hi Emmalite - Just to clarify, I was really trying to emphasise the 'up front' bit, rather than the 'cash' bit. Nothing wrong with accepting cheques, provided you lodge them promptly to highlight any chancers before their next training session.
If you are using your existing tax credits fully on your main job (salary > €29k), you will end up paying 42% tax on your additional income. The Revenue online system may be a bit daunting for someone starting out. You can simply declare your extra income on the relevant form (Form 12 I think) from the Revenue website. A call to your local tax office will point you in the right direction. Make sure you ask them about the timings of the payments, as you could end up facing bills for interest of you are late.