Just an aerial should be enough to get you free access to the basic Irish Saorview channels. Most modern TVs you buy here will support Saorview, if in doubt, ask.
It'll depend on where you're located. The Coverage Check on the site I linked will give you a good starting point, if you are reasonably close to a transmitted, an indoor aerial might do the trick.
The alternative is a pay TV service which will require more equipment and similar levels or more wiring along with installation/subscription charges.
I think its worth getting a decent aerial on the roof, or attic and connecting it to the UPC cables. The quality is much better than the signal that might be still on the UPC cables you still have.