May I offer a practical rather than legal suggestion.
Ring the tenant up explain that your mortgage costs are increasing, its been a year since the last increase, and you need to look at the rent again.
Then say that you were thinking of another €50.
The tenant may quote the RTB at you, but more likely I suspect they will see that as a reasonable increase and go with it.
Final suggestion, ask for the increase from January 'when Christmas is over' is a lovely phrase.