Smith spent three years with the Exiles before moving into the England set-up as attack coach into 2008.
His tenure within the England back-room staff came to an end after the World Cup - with Martin Johnson, Mike Ford and John Wells all leaving as well.
Smith guided the Exiles to the final of the European Challenge Cup, a Heineken Cup semi-final against Toulouse and third place in the Premiership when previously in charge, and he is relishing the chance to work with the club once again.
"I am delighted to be returning to London Irish," he said.
"Toby and I have always had a great relationship and that brought with it a lot of success on field in my time at the club and I know that we're destined for even greater things.
"We're building a new squad with fantastic potential and if we all pull together we can really make things happen."
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.