You need to register for VAT if you get over certain limits (25k euro methinks), otherwise, it's best to keep a seperate 'business' bank account, even as a sole trader, otherwise the accounts can get very messy very fast. Keep receipts, etc, for business expenses and such also.
If you want to trade under a name other than your own name, registering a business name at
Companies Registration Office will cost you thirty euro, otherwise, registering a limited company is the best route - it gives you excellent protection, but also comes with a little more work (and annual accountancy costs).