Re: pension entitlement at retirement
You need to check your scheme rules. However, I would guess, probably not. The benefit from most DB schemes is based on final salary. I am guessing that if you are reducing your working week, you are taking a salary cut (more power to you if you're not). If you finish up your employment working part time, and hence on a lower salary, your benefits will be correspondingly lower.
Revenue allow the scheme to look back to any salary in the 5 years prior to retirement, but they don't have to. The scheme will follow whatever the rules say.
It won't matter that you were paying at a "full time" rate in previous years. The whole point of defined benefit is that it is not directly related to the contributions one makes. Although defined benefit schemes usually provide greater benefits than defined contribution schemes, they can sometimes be inequitable that way.