It's great to hear that you and your group are over for a golfing trip soon!
With the 12 of you and all of your luggage and golf bags, I would say a private transfer would probably work out better in the long run. There are several companies that offer custom and private tours
which may be of help to your plans and also some Golf tour companies
who may also be able to offer useful suggestions on transport as well as courses. It would also mean that you would arrive directly at your hotel and may also prove helpful getting to the courses too!
For information on Irish Rail 's luggage
capacity, please take a look at their website as space, while substantial, may not cope with your group as well as other passengers.
Again, the Airlink Express
757 service from the airport allows for an amount of luggage but this will be in addition to all of the other passengers so it may be limited for you all.
I hope this helps get you started, I'm sure our Industry partners and fellow members will have some advice also.
Where have you planned to play during your time with us down in Killarney?