Russell only retired last month, after riding a winner at Thurles on December 18.
The 43-year-old - who won the Grand National twice on Tiger Roll and the Gold Cup at Cheltenham on Lord Windermere - had been a key part of the riding team at Gordon Elliott's along with Kennedy.
However, with Kennedy now out of action in the lead up to the big spring festivals, Russell has reversed his decision and will resume his career at Fairyhouse on Saturday.
In a statement issued on his behalf, Russell said: "After meeting with Gordon today, I have decided to come out of retirement and ride for the short period while Jack is on the sidelines.
"It's only been a matter of weeks since I retired, and I actually rode more out this morning than I have in many years.
"We are a close team in Cullentra and after what happened last weekend, I want to help the team through a difficult few weeks.
"The plan is to resume riding in Fairyhouse on Saturday and Punchestown on Sunday."