The fish, not the cross, was the first Christian symbol. Christ represented as a fish symbolizes Christ as a guide (or a link for us) through spiritual waters. From the psychic/corporeal state of being (lower waters) to the heavenly state (upper waters) as mentioned in the book of Genesis.

The Face Cross is very rare. A few are found in Ireland. This is the only one known to combine the fish symbol with the cross symbol. It is unusual and of interest because Christianity came from Ireland to Scotland. Perhaps it was placed there by Irish settlers or missionaries.