Don't worry Quarterback, it's not that complicated.
Basically, your freelance income will just be taxed as extra income. Your taxes on your main job income are all looked after for you, but you will need to settle up with the tax man each October/November for the tax owed on your freelance income.
The tax rates are the same, so if you are paying any tax at the higher rate in your job at the moment, all your freelance income will be taxed at the higher rate. You'll also need to pay 3% PRSI on the freelance income, and more than likely 2% health levy. So that could be 46% of every freelance euro earned gone - worth remembering before you sacrifice too much of your free time!
As others have said you can claim expenses, which reduce the income you pay tax on. But I'm in a similar line myself (except completely self employed) and there are not many expenses at all really. For more permanent items like a computer, these are treated as capital purchases, so you have to claim the expense evenly over 8 years - so it's of limited benefit - especially if you only pay tax at the standard rate!
Best of luck with your endeavours. Working full time and doing freelance gigs can be pretty full on, so try not to let it take over too much!
But the good news is that while you're still an employee too, you get much better tax credits and PRSI benefits - I won't start on that hobby horse
Good luck.
PS there is a good booklet available from the Revenue IT48 starting a business here: [broken link removed]