Global Radio was forced by the Competition Commission to sell around half the stations it picked up in a £70m purchase from Guardian Media Group in 2012.
Acquiring Smooth East Midlands, Smooth North West, Smooth North East, Capital Scotland, Capital South Wales, Real Radio North Wales, Real Radio Yorkshire and Real XS Manchester will hand Mr O'Brien 2.8 million listeners. Of these, 550,000 belong to Capital Scotland.
He will become the fourth largest player in the UK radio market, after Global Radio, Bauer and UTV Media. But it is understood that Mr O'Brien is keen to establish a leadership position in Britain and could do more deals and launch new brands.
Communicorp currently runs 19 radio stations in Ireland, Bulgaria, Latvia in and Jordan. The Global Radio deal requires approval from the Irish Competition Authority.
Gervaise Slowey, Communicorp's chief executive, said: "It's clear that the UK advertising market is buoyant and this move will make us the fourth largest commercial radio group in the UK from a standing start.
"The brand licence agreement that we have in place with Global for Heart, Capital and Smooth will allow us to combine the clout of a national brand with local presence."
Mr O'Brien is the largest shareholder in Irish publisher Independent News & Media.