Hi Darleith,
Sorry to say that I really didn't get much further with my research and have put it aside since then.
I didn't ever find the birth details for Letitia and the closest I could get to for her parent's marriage was the Presbyterian record for WIlliam JAMISON (a recognised variant of JOHNSON) and Martha THOMSON in Killinchy and Kilmud, County Down in 1820. The names "sort of" fit, as does the year, considering James' birth in 1823 and Letitia's in 1827 but it needs confirmation.
Since then, FindMyPast & RootsIreland have announced that they have digitised a variety of Irish records. So, it might be worthwhile checking their websites.
Let me know how you go.
Regards,
Aus_Mike