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 in account e.g. dependents or whatever).
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?
A lot of people in the software industry specifically might have significant investments, shares, maybe rental income, etc, and then you do need to specifically inform the government about those, but this isn't the case for the 90%.