The 33-year-old second row, who started his career at Leinster, has made 233 appearances for the Exiles and was club captain between 2007 and 2011, leading them to the Heineken Cup semi-finals in 2009 and the 2010 Premiership final.
Casey, who also won seven caps for Ireland, will take up a management role with club sponsors Powerday plc, and told the London Irish website: "I've thoroughly enjoyed my rugby career both with Leinster and London Irish and have lots of great memories.
"I've been fortunate to play with many talented players and work with gifted coaches and would like to thank them and the medical staff that have looked after me so well.
"My special thanks must also go to all the supporters, business and individual, of London Irish in particular, for their exceptional welcome and encouragement over the past 10 years.
"While I will miss them on the field, I hope to keep in contact with them in my new role.
"In the meantime, there are four key matches to play and I remain focused and determined to do all that I can to help London Irish secure a place in next season's Heineken Cup."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
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The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.