Further interruption was caused by backups filling storage on mastodon.ie. They've grown by about 300% since I added the relays.
As a temporary fix I've dropped backup retention to 2 days from 7. That should be more than enough, as I'm transferring them to remote storage nightly.
I missed this outtage as I was mostly offline over the last few days. Need to get monitoring in place sooner rather than later.