The second-row forward, who joined the club from Rotherham last summer, has made eight appearances for the Exiles so far this season.
The 23-year-old, who has represented Ireland at under-20 level, said: "I have thoroughly enjoyed my time at London Irish and I'm delighted to be staying on.
"I feel I have improved as a player under Toby Booth and Neal Hatley during my short time here, and I am sure I will continue to develop over the next few seasons."
Head coach Booth added: "James is a talented player who we think has a promising future. He is very physical on the field, as well as natural leader, who has performed well when called upon this 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 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.
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 All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
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.