Evermore,
Do you know what the Leith’s occupation was in Ireland? If they were farmers then they ought to be listed in the tithe applotment records. The tithe applotment records for Tamlaght O’Crilly (the parish including Inishrush) were compiled in 1833.
http://www.irishgenealogyhub.com/derry/tithe-applotments/tamlaght-o-crilly-parish.php
You will see that there were 2 Leith farms in Moneystaghan townland. James & Robert Luth. They were the only Leiths in the parish.
1831 census shows Robert was Church of Ireland. (Total of 5 in his family at that time).
http://www.census.nationalarchives.ie/pages/1831/Londonderry/Loughenshollin/Tamlaght_O_Crilly/Moneystaughan/117/
James with 6 people, 1 was RC - probably a servant.
http://www.census.nationalarchives.ie/pages/1831/Londonderry/Loughinshollin/Tamlaght_O_Crilly/Moneystaughan/2/
Again these were the only Leith households in the parish. So I would suspect one of them was your Leith family. Clearly the Robert who married Catherine O’Neill was only 11 in 1831 and so he wouldn’t be listed but his father might have been either Robert or James.
You ask what is likely to have happened to the 3 girls after their parents’ death. I can’t answer that specifically because I have no knowledge of the families but the usual thing would be for relatives to take them in. Since there was another Leith household nearby I’d guess they went there.
There are 2 townlands named Moneystaghan. One is Moneystaghan Ellis and the other Moneystaghan MacPeake. I think the townland where the Leiths lived was Moneystaghan Ellis. Griffiths Valuation for 1859 lists 1 likely family farming there. Joseph Liaf (ie Leith) had plot 2 which was a 3 acre holding. (That property today is just off the A 42 to the west of Clady.).
http://www.askaboutireland.ie/griffith-valuation/index.xml
The Valuation Revision records on the PRONI website show that it remained in Joseph’s name till sometime between 1864 and 1880 (the relevant records for the intervening years are missing). However by 1880 a James Ballentine was the occupant.
Nancy Leith, widow of Joseph Leith of Moneystaghan died 23.7.1881 aged 70.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1881/06433/4848442.pdf
Joseph died in 1875 aged 65. There was also a John Leith who died in 1876 aged 63. A Mary Leith died in 1864 aged 48. Margaret Leith, spinster, aged 46 died at Moneystaghan on 9.4.1893. The informant was her brother in law Edward McGuckian.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1893/06005/4709018.pdf
For the deaths between 1864 and 1877 the records are not on-line free yet. If you want to see them you have to look them up on the GRONI site. You can view the original certificates on-line on the GRONI website, using the “search registrations” option:
https://geni.nidirect.gov.uk
You will need to open an account and buy some credits. It costs £2.50 (sterling) to a view a certificate.
Edward McGuckin in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Claudy/Money_Staghan_Ellis/1542592/
He married Elizabeth Leith, daughter of Robert Leith, on 14.9.1890.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1890/10706/5903034.pdf
There were no Leiths in Moneystaghan by the 1901 census. In fact there were none in the whole county. They had evidently died out.
Birth & death registration only started in Ireland in 1864. Protestant marriages were recorded from 1845. For earlier events eg Robert & Catherine’s marriage or the baptisms of their children you have to rely on church records. Unfortunately the Church of Ireland records for Tamlaght O’Crilly were lost in the 1922 fire in Dublin, so there is probably no record of them. However the rarity of the name, and the fact you know they came from around Inishrush makes me fairly confident that Maoneystaghan Ellis is where they lived. You might find gravestones in the local graveyard.