The 27-year-old fly-half will join up with his new team for next season after playing for Hurricanes during this year's Super Rugby campaign.
"James signing is a huge statement of where we want to take this team next season," read a statement from Irish head of rugby operations Glenn Delaney.
"He is comfortable at both fly-half and full-back and possesses great game-management skills.
"He has been an important member of the Hurricanes squad over the last few years, and the experience he has gained from being in their environment will be invaluable to us at London Irish."
The New Zealander captained Taranaki to ITM Cup glory during his time in his native country.