Ruth,
You haven’t said what denomination the family was or which church they attended. Do you know? I’d guess they were Presbyterian, because all the McWhirters in Craigs in the 1901 census were Presbyterian, but there are quite a few Presbyterian churches in the area and so that makes it tricky finding the right one.
I had a look for gravestones. As a general rule, linen weavers wouldn’t normally be able to afford a gravestone but that’s not always correct and sometimes a wealthier relative overseas pays for one. I looked for possible graves. This is the only one with a definite link to Craigs (there are many others in the area).
1st Ahoghill Presbyterian graveyard
Erected by Robert McWhirter, Craigs, in memory of his beloved wife Jane McWhirter, who departed this life 20th Jan 1872 aged 56 years Robert McWhirter departed this life 5th Decr 1890 aged 73 years
None of the Presbyterian churches in Ahoghill has any baptism records earlier than about 1835. (Ahoghill 1sts start in 1841). The only church with records for the late 1700s/early 1800s in the Craigs area is Cullybackey (Cunningham Memorial) and they have gaps. There’s a copy in the Presbyterian Historical Society in Belfast but the records are not on-line and you have to go in person to look them up.
I think you’ll struggle to find William & Nancy’s parents from documentary records in Ireland. I doubt they exist. What about Nancy’s death certificate? Does it name her parents? William & Nancy appear to have married around 1820 (judging by the ages of all their children). Marriage records in Ireland at that time didn’t record parents names. But in any event I rather doubt that there is surviving record for their marriage (unless it was at Cullybackey Presbyterian).
I looked in Griffiths Valuation for 1862. There were a couple of small McWhirter farms beside each other in Craigs. They were the only McWhirter properties listed in the townland at that time. John McWhirter junior had plot 92a (4 acres), which included a cottage with Ann McWhirther in it. Next door on plot 93a 6 and a half acres) was John senior, and there was a small house on the farm with another John in it. John senior appears to have died 24.2.1880 aged 83, so he could be the John who is in the 1851 census married to Martha Porter. John junior died 23.9.1893 aged 69. The farms were all given up around 1899, according to the revision records. Those farms today are set back off the Craigs Rd, near Knockbane. I can’t say whether anything is still standing but you could have a look on Google Earth and find out that way.
A couple of probate abstracts from the PRONI wills site: Letters of Administration (with the Will annexed) of the personal estate of John M'Whirter late of Craigs County Antrim Farmer who died 23 September 1893 at same place were granted at Belfast to Sarah M'Whirter of Craigs Widow the Universal Legatee.
McWhirter, Matthew of Kilrea Road Craigs Cullybackey county Antrim retired breadserver died 24 May 1962 Probate Belfast 3 September to Percy Matthews hardware merchant and David Jack auctioneer. Effects £3345.
There were 4 McWhirter households in Craigs in 1901. I have no quick way of telling whether they were related to each other or to your families. However you might want to trace them back using marriage and birth records.
http://www.census.nationalarchives.ie/pages/1901/Antrim/Dunminning/Crai…
Possibly DNA testing may be a way of matching with others who have additional information about where the family originate. Family Tree DNA reportedly has more people with Ulster roots than any other company. That obviously increases the chances of finding a match. You might want to try them or, if you have already tested, you can transfer your results to them for no fee.
The North of Ireland Family History Society is running an Ulster DNA project and can offer FTDNA testing kits at a reduced price. http://www.nifhs.org (Go to DNA project on the website).