Earlier quoted context omitted.
> Their employer reports their income, deducts an appropriate amount of taxes, and sends it to the government. How does your employer know what "the appropriate amount of taxes" is? In the US I have always had trouble setting this to where I'm not at least 5% off in one direction or the other, and I know more than my employer about what my withholding rate should be.
> How does your employer know what "the appropriate amount of taxes" is? It's an approximation based on your revenue (and they know how much they'll pay you). At end-year everything is tallied all proper and as taxpayer you either have to pay the missing bits (if you have extra revenues which weren't accounted for by your salary) or you receive a wire transfer from the state (if you had deductions which weren't taken…
That still raises the question, though, of how your employer knows enough about your tax situation to not significantly over or under withhold. If my wife and I just relied on the basic IRS calculations on the W-4 and didn't specify additional withholding by dollar amount we would end up owing thousands, possibly over $10,000, the following April (before penalties). How do Ireland and others avoid that?