Because of the nature of such deals requiring commercial confidentiality between Sky and the likes of MTV, Discovery, UKTV etc. along with RTÉ, TV3 and TG4, no one in the public domain can really know what exactly is stated in the terms between the Irish (Republic) terrestrial broadcasters and their agreement to be broadcast on the Sky platform.
The only thing that is known is that Sky rent capacity from SES Astra to broadcast the four respective channels to subscribers and that there is other, non-Irish content on that transponder as well. This is unlike the UK PSBs of the BBC, ITV Network, Channel 4 and Channel 5 who directly negotiate with SES Astra and Eutelsat for capacity and then deal with Sky and Freesat for EPG placement.
Since RTÉ1, RTÉ2, TV3 and TG4 all appeared on Sky at around the same time, it may be possible that there are clauses in the respective contracts that cannot be changed unless all parties involved agree - for example, RTÉ may well wish to offer Sky RTÉ2 HD for the platform, but TV3 have a clause allowing them to object to any new RTÉ TV services appearing for the remainder of the period of the deal (before anyone starts, this is hypothetical - I am not saying that TV3 actually are blocking it!). Another one could be that Sky are interested in carrying RTÉ2 HD, but RTÉ insist that if they want to, it should be available all subscribers without the additional HD charge of €15 (as is the case in the UK with the PSB channels in HD and BBC HD) and as the current deal doesn't give specific clauses, it's deadlock.
Might also be the case that RTÉ would also want carriage of RTÉ1 +1, RTÉ Jr & News Now etc. We can only guess until press releases are made or leaks get into the wild.