The 20-year-old fly-half or centre was named in Scotland's Six Nations squad in January, believing he qualified through his Dumfries-born mother.
However, the IRB determined Shingler is eligible only for Wales after playing in an under-20 international against France in 2011.
Shingler said in a statement: "Tuesday's decision by the IRB confirms that I am now, exclusively, a Wales-qualified player.
"This has been my first senior season with London Irish and I have really enjoyed my rugby. It has also been my first experience of living away from my home in Hendy, Dyfed; this has been a fantastic life experience.
"My aim now is to play well for Irish next season and to be the best player that I can be. If my form is good enough to enable me to play international rugby in the future, then that will be great, but that decision will be out of my hands.
"Moving forward, I'm delighted to have had such a positive season with London Irish; I want to continue to show the club, management, players and supporters the respect they deserve and fully focus on my club career.
"I am very happy at Irish, I have a year left on my current contract and I hope I can play my part in a successful 2012-13 season."
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