Keirnan joins from his role as head of PR at advertising agency Sudler & Hennessey, where he established the company's PR offering.
He has also previously worked in-house at Boots, chemical company BASF and agency-side at Edelman.
Keirnan takes over from GCI CEO Mark Cater who has been leading the company's healthcare team since his arrival two years ago.
Keirnan will report into Cater and will work towards building a European healthcare hub in London.'Healthcare comms is changing and is no longer confined to just the pharma industry,' said Cater.