Alun Cathcart, executive chairman of Emap, said: "This sale, and the recent sale of Agor [its French B2B exhibition business for £57.5m], demonstrates our absolute commitment to accelerating our strategic aim of generating value for our shareholders."
The sale is conditional on the approval of Republic of Ireland regulators.
Communicorp was started in 1989 by O'Brien, who is currently in the news for his criticism of the running of The Independent by owner Independent News & Media, in which he has built up a minor shareholding.
Communicorp launched in Ireland, where it owns three Dublin stations - Dublin 98FM, NewsTalk 106FM and Spin 103.8. It has expanded to Bulgaria, the Czech Republic, Estonia, Finland, Hungary and Ukraine.
Today FM, formerly Radio Ireland, has national coverage while FM104 covers Dublin and Highland Radio broadcasts in Donegal. Today's DJs include rock star Alice Cooper, whose syndicated 3-hour rock show runs on the station on Saturday nights.