The fly half started his career with hometown club Gloucester and after making his first team debut as a 19-year-old in the 2006 European Challenge Cup game with Brive, Lamb would go on to start in the final of the same competition as the Cherry and Whites lifted their first major European trophy.
During his time with Gloucester, Lamb scored 373 points in 58 appearances, helping the West Country side top the English premiership standings in both the 2006/07 and 2007/08 campaigns.
He is looking forward to a new challenge, saying that while he has enjoyed his time at London Irish Northampton is a great place for rugby at the moment.
"Northampton as a town is very similar to Gloucester," he said.
"It's not the biggest of places but it has an incredibly strong fan base for the rugby and Franklin's Gardens is always a tough place to go and play. The club has high aspirations and want to be at the top and as an individual that is obviously great.
"I have really enjoyed my time at London Irish and am thankful to the coaches at the club, particularly Mike Catt. He has definitely had an influence on my game and helped me improve my kicking and tactical play. It has been an important part of my career, but I'm really looking forward to teaming up with the Saints from next season."
Robert, 24 joints from Welsh side the Scarlets.
Having graduated from Neath College, where he played alongside Wales stand-off James Hook, Roberts spent two seasons at the Ospreys before switching to the Scarlets in March 2008.
During his time with the Welsh region, Roberts has made 88 appearances, including 21 this season, and gathered a wealth of Heineken Cup experience. He has represented his country at every level from schools to full international, culminating in his Wales debut in the 2008 Autumn International win over Canada.
"The prospect of playing English Premiership Rugby is exciting," Roberts said.
"Northampton is one of the top clubs in England and in Europe at the moment with some top players. As a nine it is also important to play behind a strong pack, which the Saints have at the moment. The coaches are also well respected and I hope they can help me improve, which I can then show in my performances for Northampton.
"It's a massive challenge but one that I'm looking forward to."
Saints director of rugby Jim Mallinder says that both players will be valuable additions to the Saints squad for next season.
"Both Martin and Ryan are talents proven at the top level and I am certain they will be excellent additions for us," Mallinder said.
"Having coached Ryan at England Under-21 and Saxons levels we knew already what he can offer, while Martin has earned caps in a very strong and competitive Welsh national team.
"There is plenty of time for them to develop further as players and despite still being relatively young, both have extensive experience in the Heineken Cup too, which will prove valuable as we continue to strengthen our squad."