Flight UA962 from Newark to Berlin declared an emergency and diverted to Dublin.
The Boeing 767-300ER took off from Newark at 19:05 EDT, more than one hour late.
After crossing the Atlantic, while entering Irish airspace, the crew declared an emergency, asking to land as soon as possible due to an emergency on board.
A man, at seat 32L, was suffering possible congestive heart failure on board.
The aircraft landed in Dublin at 04:45 UTC and taxied to the gate, met by emergency vehicles.
The 31 year-old plane stayed more than 3 hours at Dublin before finally resuming its journey to Berlin.
Flight UA962 is expected to land in Berlin at 11:00 CEST.