UPCs different set top boxes already have had (slightly) different EPG numbers, at least in the past.
I can't remember if its still like this, but in the past the service channel was on 999 for older boxes and 100 only on newer boxes. RTÉ Two HD can completely replace RTÉ Two so the situation is not even as complicated as the way Sky did it, i.e. : RTÉ Two HD is an exact simulcast of RTÉ Two (if I'm not mistaken). They don't need to include the SD version anywhere else
On Saorview, all channels are transmitted in MPEG-4, there is no such thing as a separate "Saorview HD" box, the same boxes are used for HD and SD and so there is only one RTÉ Two HD, which you see in SD if you connect via SCART to a non-HD TV. There is no need for a separate RTÉ Two (SD) in the scenario.
On UPC things are different. HD channels are transmitted in MPEG-4, like on Saorview, but SD channels are transmitted in MPEG-2. HD channels are not viewable and are not even present on the EPG of non-HD boxes. Put simply this means that if UPC were to carry just RTÉ Two HD, SD viewers would lose RTÉ Two. That won't happen. That's why RTÉ Two HD will, on UPC, be a separate channel with a separate EPG number.