1. Where did Mary originate? Since they were married in Co. Offaly, she most likely was born and raised there. Weddings generally took place is a parish church near the bride's family (home court advantage). Using the Irish Family History Foundation's database, you can compute the number of birth and marriage rcds for the surname Littleton for each county. If you are really ambitious, you can find the numbers for each parish within a county (very tedious).
2. Females tended to marry in their late teens to early twenties. Since they married in 1823, this suggests she was born in 1802 +/- 3 yrs. Again, Co. Offaly is the likely candidate. Note - few Irish owned land. Most rented or leased land from wealthy landlords, often at exhorbitant fees. A large portion was landless - they were day-laborers when work was available. They moved about Ireland (and England) searching employment.
3. Burial? Most Irish were extremely poor. They lacked the money for headstones. Often a simple wooden cross was the only grave marker. These have long since disappeared. A parish priest may have recorded the death. However, death rcds were the most poorly kept of all. However, they may have lived to post 1863. If so, their deaths MAY have been rcd'd by civil authorities - the General Register Office. You can query this office at http://www.groireland.iehttp://www.groireland.ie.
4. Assuming you have a complete list of their offspring, the emigration of all the males to America suggests the family was landless - there was no estate to be inheirited by a son. Women were less prone to emigrate prior to the Famine. After the Famine they emigrated in almost the same numbers as the men. When they emigrated, they usually went to live with other family members - use the censuses to learn where the males lived.
5. If one or more of the women didn't emigrate, you may be able to find their death rcds in the General Register Office, or National Archives of Ireland. Another source is the LDS's Family History Library.