What is morally wrong about asking for/taking a paycut?
Assuming this is a serious question....
The way the Irish tax system is set up, any reduction in pay will reduce takehome pay. Any increase in pay can only increase your takehome (unless your query is about how pay and benefits affect each other - but you've just asked about tax).
Going on to a higher tax band will only apply for any amounts above the threshold for the band - all pay does not suddenly get taxed at the higher level. EG if the first 20K is at 20% and then it's 40% above 20K, if you earn 21K, you pay 20K @ 20% and 1K at 40% (total 4,400 leaving you with net 16,600). Reducing your pay to 20K will mean you pay no tax at 40% but your takehome is also reduced (your tax is now 20K @ 20% = 4K so net is 16K).