By law, those intermediating between owners of holiday property and tourists that rent it, so rental agencies, but also online intermediaries like AirBnB, HomeAway, Wimdu or Niumba are obliged to inform the Spanish Tax Agency about bookings arranged through them.
They will have to file a ´Modelo 179´ every trimestre, ultimately the last day of the month following that, so by 30th of April over Jan-March, etc.
The obligation was already in existence for 2018 but as the Modelo 179 didn´t become available until the 2nd half of this year, there is a one-time dispensation to declare the total of 2018 during the window 1-31 January 2019.
As of January 2019, the informative declarations have to be made every trimestre.
Info from the Tax Agency, FAQs, in Spanish.
Naturally, the informative declarations by the intermediaries will be cross-checked with the actual declarations of rental income, made by owners, either on the RENTA income tax declaration if residents, or on the Modelo 210 if non-residents.
You can file the Modelo 179 online. As of January 1st 2019 it will become available on the Agencia Tributaria website.