Unless things have changed, I would have assumed the airline would have had to put on a bus/transport to get everyone from Shannon to Dublin, which was the destination, regardless of the redirect because of weather. This has happened me many times over the years, because of fog or other issues at airports, regional ones especially. There was always a bus laid on to take everyone back to where you should have landed
That other Faro-Dublin flight was different, it landed eventually in Dublin, so the airline wouldn't have to reimburse anyone who decided to get off the flight in the UK.