Lansdowne were crowned AIL division 1A champions following their 32-25 win on Saturday over second placed Clontarf.
Earlier this year, Ronaldson brought his excellent Ulster Bank League form to club international level when he was named, for the second year running, in the Ireland Club team. The former Kilkenny College captain landed six of his seven shots at goal for a 15-point haul in Ireland's richly deserved 30 - 20 victory over England Counties.
Ronaldson will join up with Connacht Rugby for pre-season training in June 2013. He said:
"I'm delighted to have been given this opportunity with Connacht Rugby. I've really enjoyed my rugby with Lansdowne this year and I'm looking forward to the chance to develop further as a player with a club that is really going places.
"There are a lot of changes at Connacht with a new Head Coach in Pat Lam coming in and I'm excited about the move to Galway and linking up with everyone there."
Commenting on the recent signing, CEO Tom Sears said:
"Craig is a talented young player who has earned his opportunity in the professional game.
"He has consistently demonstrated in the AIL that he has all the ability to make the step up and we have no doubt he will thrive in a professional environment. He is a player we have watched closely for some time and he now has an opportunity to develop his undoubted talent and fulfil his potential."