Three separate charges come off your gross pay. Income Tax is 20% on the first €44,000 for a single person and 40% above that, then reduced by your tax credits: €2,000 personal and €2,000 employee credit, so €4,000 comes straight off the bill.
USC is charged on your whole income in bands: 0.5% on the first €12,012, 2% up to €28,700, 3% up to €70,044 and 8% above that. You pay no USC at all if your income is €13,000 or less. PRSI is 4.2% on all your earnings once you earn over €352 a week, rising to 4.35% from 1 October 2026.
Put together, each extra euro between €44,000 and €70,044 costs 47.24% (40% + 3% + 4.2%), and above €70,044 it costs 52.24%. That's why a pay rise or bonus at those levels feels smaller than you expect.