I've never seen this maker's mark before and can't trace any reference to it.
Taking into account the decoration, Harp Crowned and Hibernia marks, I'd date these sugar tongs to 1785/1786 just before the Harp Crowned appeared in an oval reserve, which occurred in 1787. Isaac Davies began his working life in 1787. Isaac D'Olier ended his working life in 1779. Jeremiah D'Olier is a possibility but as you know, it's not his known mark. Could be an unknown and unidentified mark. You could send photos of the sugar tongs and marks to the Plate Committee in the Dublin Assay office but expect to wait months for a reply.