IrishGenealogy.ie (see blogpost).
I've since received confirmation that the site was in the process of upgrading the records on the site when an error occurred, taking down a huge number of records. Without access to full details, my guess is that the upgrade involved the upload of records with a Group ID reference rather than the old style reference, and included many additional 'couple' entries in the marriage index, replacing single entries for bride and groom; the error wiped off all the existing 'old-style' reference records.
The latter have, of course, since been restored (Twitter playing an important role in the speedy corrective action) and the upgrade, as far as I can tell, was abandoned. I've now been told via official sources that the update has been rescheduled and will take place 'soon'.
The Department of Arts, Heritage and the Gaeltacht say they will publicise the update next time. Whether this will happen before or after remains to be seen.
Hopefully, some lessons were learned in the recent mishap and users of the site will be kept fully informed.