Yes, it's the same 8 to 12 week window, as it is train availability which drives the whole booking process.
You don't book on the Irish Ferries website (or Stena if you use their ships). You book with any rail company and book to Dublin Ferryport if you want Irish Ferries.
The lead rail company for Rail-Sail is Transport for Wales (TfW). I've just tried the TfW website for two sample dates of 22 January and 11 February (the latter being the 12 week limit) and all is working for me.
The ship can't sell out for foot passengers, although only selected sailings take foot/rail passengers.
In fact interestingly I've just tried for a booking from Corkickle to Dublin (as I know National Rail and most rail ticketing systems have totally the wrong train times in for Corkickle beyond next month) and TfW actually gave me the correct train time from Corkickle of an unearthly 0528 rail replacement bus as far as Millom).
If it's a weekend there may be an issue with late release of rail timetables, and hence bookings.
The port work is complete by the way.