Thank you very much for your query to Ireland Reaching Out. Congratulations on getting your family tree back that far!
Have you tried checking the Freeholders list (1746-1831) for County Down. This is available on the PRONI website (Public Records of Northern Ireland). They might be listed here.
There are also partial records at PRONI for Protestant Householders (1740) the call number is: T.808./15258
There is also a document called 'Deputy Court Cheque Book' for freeholders. It only goes from A-G unfortunately! This is also at PRONI.
I wish you every success in your research. And again well done on all the hard work so far!