The Sony Sony KDL-40EX403 is Freeview HD approved, so will be able to pick up Saorview channels. If RTÉ One is on channel 800 but showing no picture and no sound, it's possible that the channel along with RTÉ Two, TV3 etc. was added during a period of high pressure that allowed reception of Saorview from Clermont Carn in Louth, but disappeared when these conditions eased off.
On that note, it's now almost three months now until all analogue terrestrial television north and south will be switched off permanently on 24th October and with it, improved Saorview reception in the north particularly areas near the western & northern shores of Lough Neagh, and also east of the Bann. The kill switch for southern transmissions is scheduled to be 10.00am that day, will probably be in the wee hours for northern broadcasts.