No. Nissi Beach is not in Northern Cyprus. It's located in Ayia Napa, on the southeastern coast of the Republic of Cyprus—the internationally recognized, EU-member state controlling the southern two-thirds of the island.
Northern Cyprus (officially the Turkish Republic of Northern Cyprus, recognized only by Turkey) occupies the northern third. The UN-patrolled "Green Line" buffer zone separates the two. Nissi Beach sits well south of that line, about 50 km from the crossing points near Nicosia.
Famous for its crystal-clear turquoise water, white sand, and a tidal islet you can walk to at low tide, Nissi is one of Cyprus's most popular beaches—blue-flag certified, lined with bars, water-sports operators, and a party reputation that peaks in July–August. It's a Republic of Cyprus destination, fully subject to EU law, euro currency, and ROC police.
If you're crossing from the north, you'll need to pass a checkpoint (e.g., Ledra Palace or Ayios Dhometios in Nicosia) with passport/ID. Car insurance for the Republic is required—northern policies aren't valid south of the line.