What is the best/easiest/quickest way to get into Manhattan and environs from Long Island, East End or Riverhead? Is there subway or rail?

Other answer:

Rosemary:
Riverhead is on the Ronkonkoma branch of the Long Island Rail Road. The trip to Manhattan takes a little over two hours.
Gerard:
Be advised, at 12:01 AM Sunday morning 20 July, the Long Island Rail Road is threating to strike.

That means NO TRAINS will run.

Plan accordingly.

jon_mac_usa_007:
There is a train station in Riverhead for the Long Island Rail Road
Rosemary:
Thank you folks for all your input. It helped immensely!