I don't think anyone is jumping to conclusions about you.
Brendan looked at the issue in the round and mentioned the slim possibility that you could be bumped into a higher PRSI band and this lose money overall.
"I think that there might be a very narrow band where the increase brings you into a higher band fro PRSI or USC and so you might lose out by a tiny amount, but unless you are sure you are in this band I would not worry about it."
I don't think you are being difficult per se, just minding your income stream, but you talked all around this issue in your first post and Brendan went to the heart of it.
I suggest that you write to Accounts explaining your dilemma and asking them had it been their intent to reward a company employee with a pay increase that meant he had less in his pay packet afterward.
If there are intermediate levels within the company that you should use to move this forward - such as line managers or departments heads - then you should do so, but be frank about your concerns.
Management appreciates directness from employees, not cleverness per se. You'll find that a succinct parsing of your concerns will save everyone time and energy and result in a speedy resolution.
This may not be a matter that normally dealt with by the HR people in your firm and if they aren't numerate you may have to spell out your tax situation in some detail to get them to see the point.
I have to express my sympathy with you - while being one of the 85% lucky enough to both have a job AND be rewarded by your employer for working hard, you're getting less pay in your salary/wage packet.
Make sure this isn't a cunning plot by an unscrupulous employer to simply pay employees less over a short time period and seek a full accounting of the decision making process that resulted in this disincentive being applied.
Consider posting the result to AAM for others to learn from.
ONQ.