Background and info for this Aussie re Irish Ancestors - Irish Thomas Johnson married Irish Margaret Ann Jackson in 1856

Hello.  Looking for info please about my Irish roots. My great great grandparents Thomas Johnston and Margaret Ann Jackson married in Denn,  Cavan County, Ireland on 30/1/1856 and later migrated to Australia, with, I think , a child Jane , and had other children in Australia including my great grandmother Isabella Johston. Thomas apparently was born in 1828 in Ulster, Cavan and died in Australia in 1911. Margaret Ann Jackson was born in 1832 at Ulster, Northern Ireland it seems. I think Thomas'  father may have been a William Johnston?  I saw a record of a Jane Johnston born 1800 and died at Manor Hamilton Ireland who was married to a William Johnston. Anything about these families would be helpful. What they did in Ireland? Where they lived? Their Irish families? Why did they emigrate to Australia? I have an old photo of Margaret Ann Johnston (nee Jackson) with her 5 daughters (one of them is my great grandmother) and was told that  Mr and Mrs Johnston came out from Ireland in the 1850s with their 5 daughters. I don't know if all of that is correct or why. Would love to learn about both the Jackson and Johnston Irish sides of the family