Officials said they will continue to monitor the storm's projected path for a possible mandatory evacuation beginning early Friday.
No mandatory evacuation of the barrier island would be issued Thursday, officials said.
The voluntary evacuation applies to visitors and non-homeowners on western Fire Island, which is served only by ferries.
Islip Town Supervisor Phil Nolan is "strongly encouraging" Fire Island residents to leave the island ahead of Hurricane Irene, the town said in a news release Thursday.
"Although at this point we have not issued a mandatory evacuation order, we feel that it is in the best interests of the residents of Fire Island to begin to leave now," Nolan said.
That will help residents and visitors "avoid a rush of people as the storm nears Long Island," Nolan said in the release.
The release said that residents throughout the town should prepare for severe flooding, high winds and power outages. It said more information is at www.townofislip-ny.gov or 631-224-5600.