Donato's are spread all over the Philippines so it would help to know where your ancestor came from. The origin of the name in the Philippines might have been an Italian sailor but few if any Donato's would be able to trace their roots to him. Before 1800 most Chinese mestizos gave their children the same surname as the godfather. Thus 10 children from the same family might have 10 different surnames. Starting in the early 1700's the more prominant Chinese Mestizos started adapting the Spanish surname system thus my Donato ancestor, Joseph can be traced back to his marriage to Maria Rosa Sinio in 1765 in Vigan, Ilocos Sur. Her name, Sinio, indicates she was of Chinese ancestory.
In 1847 or 48 all people who did not have family surnames (which included most filipinos at thet time) were supposed to be given a family surname. Some may have gotton the Donato surname at that time.
The LDS Church microfilmed all the existing Philippine Catholic records in 1978-79 and they are a great help.