The discovery was made by a man who was carrying out gardening work at his home in Killakee, Firhouse. He reported the find to officers on Tuesday evening.
A police spokesman said a preliminary examination indicated it may have been in the ground for "up to 70 years".
They said they believed it was the skull of an adult aged between 18 and 40.
The scene has been cordoned off for examination by the state pathologist and a forensic anthropologist.
DNA testing is to be carried out in a bid to establish the identity of the remains.
GardaÃ­ said they will review missing person records as part of their inquiry.
However, they added that no link to any missing person had been made "at this early stage of the investigation".
A human skull has been found in the garden of a house in County Dublin, gardaÃ­ (Irish police) have said.