Norunway.
If Irish IFR fuel regs are same as UK you have forgotten to add 10% of calculated fuel usage and 1/2 hr loiter time at Vy which would equal about 130kgs in the Cork - rig - Cork - Shannon route. I also believe that the weight penalty for Class 1 onto the rig is 200kgs so as 100kgs of fuel is used to reach and IFR approach rig then another 100kgs needs to be taken off disposable load ex Cork. A total of 230kgs off your figs.
Using your basic figures this gives a disposable load ex Cork of approx 275Kgs or 2 workers and their bags or at a push 3 workers and no bags.