They're appealing to the public to report any sightings to 999 or 112 immediately.
Aoife Haynes Murphy was removed from Mayo's Swinford Clinic in contravention of a court order.
Gardai say two year old Aoife was taken this morning, shortly after 10am, they say they've serious concerns about her health.
She's believed to be travelling with her parents, 33 year old Paul Murphy and 34 year old Kimberly Haynes.
They are believed to be driving a black Kia Sportage car, with the registration K-E-2-1-E-N-H.
Officers believe the group are now in Dublin City Centre.
Aoife's described as a fair haired child, and was wearing a pink body suit with a checkered dress, cream tights, pink docks boots with animal print on her jacket.
The jacket has a lot of prints with zebras, monkeys, giraffes and elephants.
Gardai only issue a child rescue alert if there's reasonable belief the child has been abducted, that there's a serious risk to the health or welfare and there's enough information to supply to the public.