In a three-hole aggregate playoff for the title over the ninth, 10th and 18th holes, all three parred the ninth before Summerhays bogeyed the 10th and Kennedy and Morrison made birdies.
At the par-five 18th, Kennedy had an 18-footer for birdie but missed and watched Morrison, who had three-putted the 18th for a bogey six when he had the title in his sights in regulation, made birdie to become the first American winner of the title.
Nolan closed with a level par 72 to finish 11th on four-under while Roganstown’s Sean Keeling was tied 36th on two-over after a 71 and Royal Dublin’s Hugh Foley 68th on 11-over after an 80.
Kennedy was hoping to become the fifth Irish winner of the title after Warrenpoint’s Paddy Gribben (1998), Hermitage’s Stephen Browne (2001), North West’s Brian McElhinney (2003) and Holywood’s Rory McIlroy (2006).
Scotland’s Calum Scott, who finished two shots outside the playoff in solo fourth after a closing 68, joins Morrison in The Open at Royal Troon after he won The Open Amateur Series order of merit following his performances in the St Andrews Links Trophy, the Amateur Championship and the European Amateur Championship.