The Toffees centre back, who spent last season on loan in the Championship with Yeovil Town, was delighted to pick up his first senior cap
A former captain of the Ireland Under-21 team, Duffy's first involvement with the senior international squad came in May 2010 under former manager Giovanni Trapattoni. However, that call up almost ended tragically as the Everton defender suffered a laceration to his hepatic artery, which required him to undergo life saving surgery.
Four years on from his near-death experience, the Derry native views his senior bow as a type of "reward" for the hard work he had put in since enduring the injury. Speaking to reporters ahead of Ireland's clash against Portugal, Duffy said that it was "a dream" to be selected by Martin O'Neill.
"It's been a long four years," said the 22-year-old. "I don't know, I was just focusing on club football to see if I could get back to where I was, and hopefully this season I have shown that.
"Playing the other night was sort of a reward for four years ago, what happened, so I was just delighted. "I am made up to be even in the squad, so to play was a dream.
"Obviously when you start playing football, you want to play for your country, so I was delighted when it actually came.
"I thought I did all right in the game, some good things, some bad things, but it's the first step and hopefully I can do better stuff."
The Boys in Green take on Portugal in their final summer friendly fixture and Duffy is hopeful that he will be selected and says he will be ready if called upon.
"As soon as you have played one game, you want to play more," he added.
"I don't know how I did [against Costa Rica], but I just went out there and did the best I could do, and on Tuesday [against Portugal] I will be ready if I'm picked."
Meanwhile, Martin O'Neill will be without the services of Glenn Whelan for the clash against Paulo Bento's side after the Stoke City midfielder was granted permission to leave the squad for personal reasons.
However, Wolverhampton Wanderers defender Stephen Ward re-joins the squad following an injury sustained by Marc Wilson during the Costa Rica encounter.
Republic of Ireland face Portugal at 00:45 BST on Wednesday June 11 at MetLife Stadium, New Jersey.