Google DoubleClick was receiving data from eight apps tested, while Facebook was receiving data from nine.
Ireland's data watchdog has launched 19 investigations since GDPR came into force.
If found guilty, it would be second time a European regulator has taken action against Google.
A cookie-less world awaits...
Google 'did not properly obtain consent' from users
Breach dates back to before Marriott bought Starwood group in 2016.