Kieran (above) will join Cirkle on 28 February. She has been in her current role at Text100 since the middle of last year when consumer specialist Lexis was merged into its Next 15 sister firm.
Kieran had joined Lexis in January 2015 from Kindred, and has experience working for brands including Pizza Hut, Miele, Costa, Harley-Davidson and Microsoft.
Kieran will become part of the five-person board overseeing 40-strong Cirkle.
Cirkle chairman Caroline Kinsey said: "With her exemplary new business win rate and a proven track record in exceeding KPIs, we look forward to Ruth spearheading our continued new service innovation and growth trajectory."
The agency is listed at number 77 in PRWeek's UK Top 150 Consultancies table, with revenue of £4m in 2016. The Buckinghamshire-headquartered firm won the Best Agency Outside London category at the 2017 PRWeek Awards.
Previous Cirkle MD Ruth Allchurch left the firm last year, and this morning was announced as the new consumer brand managing director at Cohn & Wolfe.