Scrum-half Allinson, who represented Wales at Under 16, Under 18 and played for his country in the Under 19 World Cup in Dubai in 2006, was delighted to have pledged his future to the Exiles.
"I have really enjoyed my first 16 months at London Irish and the opportunity to stay here for another two seasons was something I could not turn down," said Allinson.
"I have been playing my best rugby and with the help of the coaching staff I have pushed my game onto the next level."
"It is great to have such competition for places and it motivates me to be the best I can be.
"Paul Hodgson is a great player and with Ross Samson also coming in with positive performances I'm really being kept on my toes."
London Irish head coach Toby Booth, who masterminded his side's impressive Heineken Cup victory over Racing Metro 92 in Paris recently, praised the Welshman's contribution to the side.
"I have been very impressed with Darren's work ethic since he joined the club," said Booth.
"He has worked hard from day one and has earned the right to be a regular in the starting fifteen."
Educated at the famous West Wales School at Gwendraeth, Allinson successfully came through the Scarlets' Academy. Prior to joining the Exiles he spent four seasons at Cardiff Blues.