Finally RTENL have released more info. The main discussion is on the Saorsat sticky.
Regarding single dish feed this is the size recommendations:
Dark green 80cm at prime focus, light green 100cm at focus.
Regarding dual feed this is the design recommended by RTENL
For a reliable SAORSAT service, because of its frequency and the different locations of satellites, the SAORSAT Ka LNBF must always be directly on the end of the support arm and pointed directly towards the centre of the dish. LNBF’s for other services such and ASTRA / UK Free-To-Air can be offset to the left and slightly elevated, as shown above. With this arrangement services from both satellites can be reliably received.
If the SAORSAT LNBF is offset away from the centre of the dish (to the right and lower down) the services will not be reliable
Based on this info
I can't see how the 28.2 LNB will get enough signal. Something does not add up
In my opinion it would be best to offest the Saorsat LNB a little bit, in order for the 28.2 LNB to see the equivalent of a 60cm dish.
I would recommend using an 80cm dish and mounting the Saorsat LNB off by 5 degrees, and mounting the 28.2 off by 15 degrees.
The dish shown in the photo looks to be the Inverto 80cm dish (as supplied by onwave) with a special offset arm, which is not yet publically available. It seems to use a washing machine hoseclip type holder for the second LNB
I would not recommend Inverto/Onwave dish as it seems as if the 28.2 LNB will not get enough signal, and it does not appear to have the ability to offset both LNB's
The triax TD78 with dual mount seems to be a better solution although for safety an 88/90cm dish would be a safer bet. Alternatively the multifocus dish although it will probably work out more expensive.
For the extreme South West/North Donegal an even larger dish may be needed.
By the way the Triax unique dish is no longer manufactured.