Harkin sold his stake in RJH Public Relations to Wessex after a News of the World report, for which a reporter posed as a sheikh, alleged the pair were exploiting her royal connections.
After the scandal broke in 2001, Harkin quit the industry to study for an MBA in Australia.
He returned to the UK in 2003 to set up a PR division at celebrity and events agency TFA Group.
He then founded his own agency, Bikini PR, seven months ago.
'I will use my celebrity contacts to add a lifestyle dimension to Brighter's travel brands,' said Harkin of his appointment.
The Bikini clients that Harkin will take to Brighter include Flemings Mayfair, Down Hall Country House Hotel and publisher Terence Conran.
He leads a five-strong team and reports to Brighter PR managing director Steve Dunne.