In a nutshell, we may never know. My research has turned up nothing to reveal how Tom Clancy named his character. Still, there are similarities that might cause us to wonder:
* Clancy’s Jack Ryan, like ours, was Irish Catholic. Of course, the same thing can be said of Tom Clancy. It seems just as likely that Clancy gave his character a background with which he was familiar - - to a point. Tom Clancy never attended a military academy or worked in military intelligence.
* Clancy’s Jack Ryan graduated from the US Naval Academy, while our Jack Ryan graduated from Officer Training School at the US Coast Guard Academy.
* Clancy’s Jack Ryan, like ours, was "highly skilled, disciplined, honest, thoroughly professional, and only lose their cool when incompetent politicians or bureaucrats get in their way." (Hixson, Walter L. "' Red Storm Rising': Tom Clancy Novels and the Cult of National Security" Diplomatic History, p 606. Reprinted in Tom Clancy, Wikipedia. https://en.wikipedia.org/wiki/Tom_Clancy).
* Our Jack was thoroughly professional and enjoyed sharing his craft with others. He taught Hawaiians how to work on both sides of the camera. He recited poetry, both on screen and off. He even studied to become an art teacher at New York University and ran an art school with his brother while they were in college. Clancy’s Jack, of course, went on to teach at the US Naval Academy.
Here, we might wonder whether Clancy didn’t model his character on our Jack’s character, Steve McGarrett. Clancy’s first novel, The Hunt for Red October, was published in 1984. In all likelihood, Clancy was planning, researching, and writing it while Hawaii Five-0 was still in production.
* It has been said that McGarrett was an amped-up version of our Jack. He definitely lost his cool when the governor and Jonathan Kaye got in his way. Our Jack, in his early personal life, was known to get into fights, even to the point of fighting boys four years older than himself when they stoned a young bird to death.
* Clancy’s Jack Ryan’s techniques weren’t so very different from McGarrett’s techniques in the Wo Fat episodes of Hawaii Five-0, especially the in-depth ones: Nine Dragons (Season 9), Year of the Horse (Season 11), and Woe to Wo Fat (Season 12).
Something tells me that, if the writers had presented McGarrett with a pretty young daughter, he’d have been just as smitten as Clancy’s Jack was with his daughter, Sally, in Patriot Games.
Oh! By the way, Harrison Ford, who portrayed Clancy’s Jack Ryan in Patriot Games and Clear and Present Danger, was considered for the role of Steve McGarrett in an early script for the 1996 movie version of Hawaii Five-0.