Ireland boss Farrell will oversee November encounters against New Zealand, Argentina, Fiji and Australia in Dublin before temporarily departing his post in the build-up to leading the 2025 Lions series away to the Wallabies.
Easterby, currently Ireland's defence coach, will take the reins from December 1 and be responsible for the national side during the 2025 Six Nations and a post-season tour, details of which have still to be confirmed.
"To be asked to lead the Ireland team is a huge honour and one that I am thrilled to accept," the 49-year-old Easterby told the IRFU website.
The 65-times capped former Ireland flanker added: "It is the highest achievement of my coaching career to date and I'm looking forward to working with the talented backroom team and players while Andy is away with the British and Irish Lions."
Easterby joined the Ireland coaching team in 2014 and will next month manage the Emerging Ireland squad's tour of South Africa, having previously performed that role in 2022.
"I have greatly enjoyed being a part of the men's national team journey over the last 10 years and I believe that there are even brighter days ahead as we build towards a huge few months," said Easterby.
"There's a lot to play for before December and we're looking forward to providing the next crop of future Irish players with a chance to impress against talented international opponents in South Africa and then across the four Autumn Nations Series matches at Aviva Stadium."
IRFU performance director David Humphreys said: "I am delighted to confirm Simon's promotion later this year and it is recognition of his evolution and high-standing as a coach within the Ireland set-up over the last 10 years."
Former England international Farrell has guided Ireland to successive Six Nations titles (2023 and 2024), as well as victories over all of international rugby union's leading teams, since replacing Joe Schmidt following the 2019 World Cup.
His Lions team will now face an Australia side coached by Schmidt in three Tests on July 19 (Brisbane), July 26 (Melbourne) and August 2 (Sydney).
