Broadcasters have to pay for the rights to use the DAB transmitters.
RTE won't pay to use transmitters in Wales because their funding is specifically for broadcasting to Ireland.
Irish commercial stations would only pay for transmitter access if they could offset the costs by attracting advertisers who wanted their commercials to be heard in Wales.
Even if RTE or Irish commercial broadcasters could find the funding to transmit in Wales, they would have another hurdle to overcome. Nobody can transmit radio programmes in the UK without a licence from the government. Companies pay very large fees (running into many millions of pounds) for the rights to transmit in a certain area. An Irish broadcaster would only be permitted to transmit in an area already allocated to other licence-holders if their programme content (and therefore their target audience) was totally different to that of existing broadcasters. (Otherwise, the Irish firm would be able to take advertising revenue away from the existing broadcasters who had paid large fees to be able to acquire such revenue).
You can check which stations should be available in your area here:
http://www.bbc.co.uk/digitalradio/
Chris