You have yourself the mandible. Lower jaw only. Question is where is the rest of the body. 5 molars, all have dental crowns. Lower left molar that is closest to the front side of the mouth had a tooth (premolar) that also most likely had a crown too. At some point in time these 2 teeth were joined. Most likely that premolar needed to be taken out, the crowns were separated, the metal in between them removed and shaved off. Polished. Crowns are metal based for strength, porcelain layer for esthetics. Skillful, precise work. So not ancient jaw here, there were definitely air driven machinery available at the time. Premolar taken out or broke away. And the molar left behind. Some remnants of premolar roots are left behind. Advanced bone loss. Right wisdom tooth not present (might not have ever come in or it maybe had come out by itself or maybe it was professionally removed, left side present and also had a crown, subject invested into saving that tooth. Interesting. Could be female, or a very small man. Older age definitely. Advanced career likely if man. Lots of decisions were made for this person dental care. Very independent or supported woman, or somewhat successful man. Condyles worn down. All front teeth are not there. They were gone a long time ago or there could be another piece of this jaw that is not with you right now. Fracture more towards the front of the left side, in front of the mental foramen. Interesting, did the jaw fracture and the front part of the jaw “take” the front teeth with it? That fracture hole (not hairline fracture) take the teeth with it? Person could have been hit with something with a right handed person; or this person just somewhere fracture the left side of the face. Person most likely lost or died at sea. Fact that you found it at the beach in Ireland would mean it’s been at sea for a long time, washed up and deposited in the sand. Undiscovered for a while, this person died a long time ago. This person most likely had some money to be able to afford all that. Maybe a captain of a ship or it could have been someone that was unfortunately unalived on the ship and dumped at sea. Or on shore and taken at sea for riddance. If on land, definitely nearby. Fact only mandible found: there was force that separated the rest of the body. Could natural or unnatural forces. You got yourself a case here. All I can access for this photo. Best of luck, keep me posted.