The digital TV tuners in TV sets are not compatible with NTL digital. You'll always need an external box supplied by NTL for that.
RTE is conducting trials on a digital TV system which may be compatible with the digital tuners found in some sets, but then again, may not. The present situation is that the four basic channels can be received, in places where the trial is taking place, using the UK-type digital tuners in these sets. These use what is now, an out-dated MPEG2-based technology. It was state of the art when the service started in the UK.
But there are other channels being tested by RTE for which a special external box is needed. Not least because they're encrypted but also because more modern MPEG4-based services are also being tested.
The official digital service whenever it is released, might be based on the existing digital tuners, or might be based on completely new technology. Politics, both public and commercial will affect the decisions and timing.
So, when buying a new TV, a digital tuner is 'nice to have' but not a deal-breaker.