Kiernan makes the switch to the Barclays Women's Super League 2 outfit for the remainder of the 2025-26 campaign.
A Republic of Ireland international, Kiernan had made seven appearances for the Reds across all competitions this season.
She joined Liverpool from West Ham United in 2021 and penned a new contract with the club ahead of the current campaign.
Kiernan now links up with Forest, who are next in action against Birmingham City on Sunday (February 1).
