To answer my own question, #RichmondGate always shuts at 18:30, usually opens at 08:00, but at 11:00 on Sundays and Bank Holidays. Except for Good Friday and 3 days around Christmas when it is just plain closed.
FWIW, this is all the same for the IMMA grounds front gate.
Opening hours for IMMA itself are here: https://imma.ie/visit/info/#opening-hours.

#IMMA #IrishMuseumOfModernArt