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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Lions get ready for their matches with this green training jersey. It's what the players wear. Get in the Gear!
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.