UTV head of press and PR Orla McKibben said Powerscourt was hired without a pitch, bringing to an end the two-year relationship with Stakeholder Communications.
The account is led by Powerscourt co-founder Rory Godson and former Finsbury associate partner Anthony Silverman, who joins Powerscourt on 20 June.
UTV has made an agreed £100m bid for The Wireless Group, whose chief executive is former Sun editor Kelvin McKenzie.
Bell Pottinger Corporate & Financial director David Rydell has advised The Wireless Group for the past four years. However, The Wireless Group's imminent delisting following UTV's acquisition would terminate Rydell's relationship.
Beyond communicating the merits of the buy-out to investors, UTV is likely to use Powerscourt to position the company as a mid-cap media group rather than a provincial broadcaster.
TalkSport would join UTV's other radio stations in Liverpool, Dublin, Cork and Limerick. UTV also holds the ITV franchise for Northern Ireland.
Silverman is the third mid-ranking PRO to quit Finsbury in the past two months.
Associate partner Alex Shorland-Ball quit in April to join Lonmin as V-P of IR and comms. Executive Nicola Hobday, meanwhile, has joined Holloway & Associates as a director.