A combination of Shannon, Brest and Madrid radars provide complete coverage for T9, up to what altitude I couldn't say, however VHF comms. are not always possible for the entire route. (Brest will on occasion have VHF problems west of 008W depending upon conditions, therefore the same 'overlap' achieved by radar isn't always possible).
There was talk a while back about trying to establish ADS-B ground stations in the vicinity to enable a surveillance service to be provided by ATC, in addition to the procedural one provided at present, but I believe the planned introduction of SATCOM-based ADS-B in the NAT Region has overtaken such proposals.