Interest is credited quarterly.
So if you lodge €10,000 you should get €101 approx net of tax after 3 months, and further similar payments thereafter.
Note that once this happens, the balance above the €10,000 threshold will then be earning 4% so strictly speaking you should transfer the interest payment out into a more competitive account to maximise your interest.
The 5.15% rate is a special offer for one year only - so yes, the rate reverts to the prevailing standard rate on the full balance after year 1 (currently 4%)