I cannot recall seeing the word ‘Registered’ on a headstone previously.
Maybe some burials were not registered with the church/cemetery authorities.
Maybe not all deaths were registered – or even all births.
Another item has gone onto that ‘TO FIND OUT MORE’ list……..
We see if fairly regularly throughout the country - it means they bought and registered the plot with the powers that be - usually the COI.
I think in Garrankennefick (nr Aghada, Cork) there is a "Registered and three foot on either side"