Hi Paul,
I just did this last week myself - I'm PAYE, but am starting to do some other work on the side.
1. Download IT48, Starting a Business, from the revenue.ie website. Read it - it's very helpful.
2. Download IT49, Guide to VAT, from the revenue.ie website and flip through it. Basically if you are providing a service or adding value in any way to an item you then sell on, VAT will be involved, so you need to know how much to charge and what to do with the VAT money. However, you don't need to register formally for VAT unless you're making over E25.5k for selling a service or E51k for selling goods per annum.
3. Download TR1 form from the revenue.ie website. Fill out section A to register yourself for income tax (and VAT, if necessary) and send it off.
4. Download last year's Form 11 (tax return) and the Guide to Form 11. Figure out what all parts of section B mean, as that's the part where you'll declare your income from your hobby (your PAYE income goes in section E). Then study all the rest of the form to find out how to claim as much relief as you can!
5. You'll need to submit your Form 11 by 31 October every year for the previous year's income. Submit it by the end of August if you want the Revenue to do the sums for you - they'll send you a tax demand and you just take it to your bank and stump up. Depending on whether VAT applies, you may need to submit VAT forms and cheques every two months - the Revenue will send you the forms themselves if the amount you think you will be making warrants it.