The New York Shamrocks were an original member of The League, one of four founding members in 1916. The Shamrocks were also the first champions of The League, having won the championship that year. When The League returned in 1923 following World War I, the Shamrocks returned. That year, the Shamrocks were part of several league milestones, playing the first game of the season against the Milwaukee Bottlers, and having that game broadcast over the radio, the first football game to do so.
In 1935, players on the Shamrocks were involved in the "Highland Boulevard Donnybrook," a brawl with players from the New York Dutchmen in Marshall Cooper's rival league. Players on both sides were arrested, and Cooper and Hugh Walker, then-Commissioner of The League, agreed that the two teams would play with ownership of both leagues at stake. The two teams met on New Year's Day of 1936. The Shamrocks controlled the first half, leading 18-6 under Walker's organized, penalty-heavy rules. The second half was played under the open, free-wheeling rules of Cooper's league. The rule change allowed the Dutchmen to overtake the Shamrocks and win 42-21. The Shamrocks, however, would survive the World War II-era contraction, while the Dutchmen would not.
The Shamrocks appear to be one of the more successful franchises of the six-team era, boasting players such as linebacker Chuck "Skullcrusher" Koswolski (who would eventually go on to be the head coach of the Chicago Marauders from 1983-1987), who used his helmet to spear opposing quarterbacks, causing players in both the Eastern and Western League to consider taking advantage of the optional helmet rules. In 1965, the Shamrocks hosted the Los Angeles Lightning in the first-ever exhibition game between the two leagues. The Shamrocks came out victorious 51-45.
Two years later, the Shamrocks, as Eastern League champions, traveled to Sacramento to play the Western League champion Sacramento Cyclones in what was to become a tradition. Unfortunately, after the Shamrocks won the game, 41-31, tragedy struck. As the Shamrocks entering New York, their plane went down in a electrical storm, killing every player and coach on board. Only three Shamrocks players Frank Tiberon, Mark Jensen, and All-Pro Defender Charles Jenkins III who did not make the trip due to injuries, survived. An infamous picture of the wreckage was featured on the front page of the New York Herald, beneath the headline, NY NIGHTMARE! The city of New York would be without football for nearly a decade, until the inception of the expansion New York Nightmare in 1979.