A Form 11 is for people who are fully self-assessed, or who wish to move into that category. People who are currently PAYE, and whose net taxable additional income is more than €3,1xx (missed the fine detail, but the three thousand one hundred bit is right), must return a Form 11.
A Form 12 is for people who are primarily PAYE, and whose net taxable additional income is less than €3,1xx This offical also informed that I can make a note with my Form 12 to the effect that I would prefer to make any payment, or take any refund through the tax credit system. This I am going to do.