I presume the employer ( public sector) would have to make up the shortfall, via the supplementary pension.
The supplementary pension is paid to public sector workers, in your wife's scheme, if their social welfare payments do not bring them up to the full value of their final pension.
So, basically, your final calculation is correct. However, she would have to apply for three seperate pensions.
The Public Sector pension, the Contributory State Pension and the Supplementary pension. She is only entitled to the supplementary pension when she has exhausted her social welfare entitlements and still doesn't hit the 50% of final salary target.
Another point to remember is that when she job shares, she will be, effectively, reducing her number of years worked for the public sector pension.
Each job share year, only counts as half a year of pensionable service. So, depending on how many years she has to go, it might be difficult to hit the full 40 years of service.
I'm surprised that part time workers are punished in this way. Its not so bad for your wife, as she can claim a supplementary pension from her employer to make up the difference. But job sharing employees working one week on, full time, and one week off, seem to be at a significant disadvantage to those working 2/3 days a week.
Explains how part-time work can affect your entitlement to social insurance payments.
www.citizensinformation.ie