Jarvis enjoyed a brief spell with Leeds at the end of last season after playing for Bristol in the Championship.
The 27-year-old brings depth to the Exiles' fly-half options after Ryan Lamb left for Northampton and Chris Malone retired at the end of last season.
London Irish head coach Toby Booth said: "During a World Cup year when there are key players missing, it is important to have players around the squad with experience and that is something Adrian will bring to us.
"We have talented youngsters who will learn a great deal from him. His kicking out of hand and at goal is outstanding and it is very good to know that we will have that in our locker for 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.