Tax Exempt Due To Low Earnings?

I earned 8100 in 2003 as a sole trader, I filled out my Revenue Online last night and it says I am exempt from paying any tax? Is there a minimum you have to earn to be liable for tax? I thought I'd have to pay tax on at least some of it.

Thanks,
OweTax.

Found answer

Income 8100
Tax @ 20% 1620
Less single person tax credit 1520
Tax liability 100

Must be some other tax credits I'm not aware of that covered the last 100.

Not sure if the rules for self employed are different to PAYE employees but this link outlines the tax exemption income limits in various circumstances:

[broken link removed]