The 33-year-old has agreed a one year deal with the province.
Speaking today, Leinster Rugby Chief Executive Michael Dawson said: I'm pleased to welcome Guy to Leinster.
"He brings a wealth of experience and provides greater competition for places at scrum-half. I am confident that he will be a valuable addition to our squad.
Born in Harrogate, Guy Easterby was educated in Ampleforth College and played for Yorkshire and North of England Under-21s. He played for Harrogate until 1996 when he moved to Rotherham.
He since had spells with London Scottish, returned to Rotherham for his second spell, Ebbw Vale (where he earned a total of 2 Ireland caps) and Llanelli Scarlets (14 caps) before rejoining Rotherham (10 caps) last year.
Guy has made six starts for Ireland and earned the last of his 26 caps during Ireland's 26-17 second test defeat to South Africa in June.