Many thanks for your post to the Ireland Reaching Out message board and welcome to the site!
You mention the Gravestones of the Omey, and there is a book that appears to list many of the headstones located within that particular area.
It is available to purchase and the website with the information for that is available at the link below:
As to your query about a better birth date for Joseph or Letitia, that may be difficult to find if records are not available. You might consider looking through the parish registers that are available online at the National Library of Ireland website.
The link for the parish of Termonmacguirk is listed below:
The best of luck with your research!