You will need to register a business name if you intend to trade under any name other than your own. As you are already aware, VAT registration is not neccessary unless you earn over the thresholds, but if you are dealing with businesses only, then it may be beneficial to register anyway so you can claim VAT back on any business expenses.
As for Income Tax, anything you earn extra will need to be declared to the Revenue (you, or an accountant, will need to file this form yourself!) and this will be added to your PAYE earnings on your P60 at the end of the year. You will then pay IT in the normal way, i.e. up to €34k @ 20%, anything over @ 41%. You will also need to pay a certain amount of PRSI yourself according to your extra earnings.
This is how I operated for approx. 2 years before I left my job and went full-time. It's a good way of testing the waters before committing to it.
Best of luck...!