Brian MacLaurin, who first started his own PR business 21 years ago, will continue to operate under the MacLaurin Media name, handling reputation management for a handful of undisclosed clients that include law firms and business people.
Johnson, who has been running MacLaurin Media's consumer division, will own and head Insider Communications. She will continue to handle a number of MacLaurin Media's consumer clients, but declined to reveal their identity at this stage.
Iredale will own and run Iredale Communications, which will take on a number of MacLaurin Media's corporate, health and trade clients.
The new agencies are expected to launch in early summer, when they will announce their individual client bases.
They will employ current MacLaurin Media staff.
MacLaurin Media's current client base includes King of Shaves and Global Radio properties Capital FM, XFM and Classic FM.
The agency also manages PR for individuals, including DJ and presenter Dave Berry, entrepreneur Sara Murray and Russian billionaire Andrey Melnichenko.
MacLaurin will be contracted as a consultant for both Johnson and Iredale's new comms agencies.
He said: 'My plan is to consult for these two businesses, to let them develop and flourish, and to do my own consultancy work.'
He launched his first agency in 1992 as MCM, which was renamed MacLaurin three years later. The agency was sold in 2001 to a venture capital-backed management team.
In 2003, the PR veteran launched an agency, Brian MacLaurin Associates, which he renamed MacLaurin Media three years ago.