My understanding is as follows:
1. You must complete a Form 11 if your Deposit Income is treated than €5,000
2. Any Deposit income declared on a From 11 is liable to PRSI
So, if you earn €3,000 in Deposit income, that does not force you to complete a Form 11. However if you complete a Form 11 for other reasons, and enter €3,000 Deposit Interest on it, that will be liable for PRSI.
I am not certain about this, however I believe it to be true. Happy to stand corrected if someone with authoritative knowledge of this can factually correct my interpretation.