Ion joined Irish from Bath in 2009 with his debut coming at Twickenham in the London Double Header against Saracens. He's gone on to make 64 appearances for Irish, scoring one try.
He made his international debut against Argentina at the 2003 Rugby World Cup and has also featured at the 2007 and 2011 World Cups.
Capped 18 times by Russia, Kulemin joins London Welsh from Castres after four seasons at the French Top 14 club.
The 31-year-old had previously spent four years at Agen, with whom he played Heineken Cup and Amlin Challenge Cup rugby, before joining Castres in 2008, where he gained further European experience.
Kulemin began his playing career in rugby league with Dynamo Moscow, who in 2003/04 became the first Russian club to compete in the English Rugby League Cup.
His international rugby union debut came in 2006 against Portugal and he helped Russia qualify for last year's Rugby World Cup in New Zealand.
"They are two fine signings who will help our squad to perform to its maximum," said London Welsh Head Coach Lyn Jones. "They are boys with lots of experience at the highest level in the English and French leagues and can bring the best out of other players."
London Welsh kick off their Amlin Challenge Cup campaign with a glamour home tie against Stade Francais at the Kassam Stadium on Saturday, 13 October.