The threshold had originally been set at 10,000, but was doubled after political pressure.
Homeowners in four Kildare towns can continue operating Airbnb-style rentals without falling under new national restrictions.
Maynooth, Leixlip, Athy and Kildare town are all below a new 20,000 population cut-off agreed by Government.
That means the tougher rules - which would have required planning permission and registration for short-term lets - will not apply in those towns.
The threshold had originally been set at 10,000, but was doubled after political pressure.
Critics say the change weakens efforts to push more homes back into the long-term rental market.
The Green Party says changing regulations on short term lets is pointless unless the rules are actually enforced.
It's expressing disappointment at the decision to introduce restrictions for areas with a population of 20 thousand people and not 10 thousand as originally proposed.
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