Hugh MILLIGAN

I’m researching my maternal 3x great grandfather Hugh MILLIGAN and his family, and could use some help!

Hugh emigrated from Ireland to New Zealand in 1858 as an assisted emigrant aboard the ship ‘Clontarf’.  The ship's passenger list shows Hugh aged 44 and his wife Margaret aged 42 from Ireland. The children were:
Hugh (21)
James (19)
Sarah Jane (17) - my 2x great grandmother
Thomas Batt (15)
Elizabeth Otway (14)
William John (11)
Isaac Henderson (8)
Mary Ann (6)
Margaret Hood Clarke (4)
Niece Elizabeth Milligan (22)

Margaret was Hugh’s second wife. They married in 1851 and I have obtained a transcript of the marriage -  “At Kilmacrenan, on Monday, the 20th inst., by the Rev. Henry C. Cochran, Hugh Milligan, Killygarvan, farmer, Widower, son of Hugh, Farmer, married Margaret Hood Clarke, Drumhalla, dau of John, Farmer, 20 Jan 1851. Witnesses David Clelland and Grace Henderson.”

I looked up Killygarvan out of interest and to add some context to our family story, and came across mention of the Batt family – Thomas Batt of Rathmullan and his daughter Elizabeth (Batt) Otway.  Given that two of Hugh’s children from his first marriage are named for (or at least appear likely to be) Thomas and Elizabeth, I now feel there may be a connection to the Batt family which I’d like to explore.

The occupation of Hugh Milligan (both father and son) has been recorded as ‘farmer’ so perhaps they were tenants at Rathmullan?  Are there any historical records for the property that would be able to tell us if this was the case?  Or is there any other way to find if there was any other connection?  

I've also seen reference to a Hugh Milligan being a cess-payer in the barony of Killygarvan in 1842 and 1847 - I'd be interested to understand exactly what this means.  Could this have been "our" Hugh Milligan or his father?

Another mystery is the name of Hugh's first wife, which we have so far not been able to find.

The death certificate for my 2x great grandmother Sarah has no mother recorded, and I have been contacted by a descendent of Elizabeth Milligan who said her death certificate shows her mother was Elizabeth Otway.  I’m now questioning this, given the coincidence.  It may be that the daughter who reported the death has assumed Elizabeth was named for her mother, as she also did not know her father’s name (odd, given that one of Elizabeth’s sons is named for Hugh).

Is anyone able to suggest where I might look next to see if I can find out anything to prove or disprove my theory? Unfortunately we don't know if he was even living in Donegal at the time of the marriage or the birth of his older children, so it could have been elsewhere. 

Searching from New Zealand for any historical information about the family in Ireland has proven quite difficult, as we have very little detail about them or where they came from in Ireland.  So any assistance you can provide in finding a record of Hugh, his first marriage and family, or any other help in locating the family would be very much appreciated.

Thanks

Catherine