Hi Fanny, from Penn station (midtown Manhattan), the best thing is to take train 2016 to RONKONKOMA station which is very close to Long Island MacArthur airport (ISP). It will take you 1h23min! Then, maybe you can take an UBER from the station to the airport, which is about a 12 minute drive! The Ronkonkoma train also departs from other places in Manhattan. All the best!
Suffolk County Transportation S57 bus connects to the Long Island Rail road at both Sayville and Ronkonkoma Station. From Sayville Station take the Montauk line to Babylon station and connect to the Babylon train. Babylon and Ronkonkoma trains connect to the subways at Atlantic Terminal in Brooklyn, New York Penn Station and Jamaica Station. There is also a bus called the Hampton Jitney that serves the east side of NYC and stops near the airport
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