The Irish Aviation Authority has said it inadvertently allowed registered drone owners to access the names and contact details of all other registered users.
It says this was due to a technical problem with the register which has now been rectified.
All drones in Ireland weighing 1kg or more have to be registered with the IAA.
There are estimated to be between 4,000 and 5,000 drones already in use in Ireland.
The IAA says no financial information was stored on the register, and that the system was not hacked.
Only registered users could access the database and there was no unauthorised access by the general public, it says.
In a statement, the IAA said it "apologises to all those on the register for this problem and assures them that it has been resolved and will not happen again".
It says it has informed all drone owners on the register of the issue and has been in contact with the Office of the Data Protection Commissioners.
The IAA has also asked anyone who unintentionally accessed the information to destroy any material they may have downloaded, in accordance with the Data Protection Act 1988.