You haven’t said what precise denomination your Watt family was. From the information you have provided, I would guess either Presbyterian or Church of Ireland. The Church of Ireland in Donaghmore has records from 1758 onwards; the Presbyterian church in Carland has records from 1759 (with gaps) and Castlecaulfield Presb church has baptisms from 1855 and marriages from 1834. There are copies of those records in PRONI (the public record office) in Belfast. You may be able to verify or extend your information from that. A personal visit is required to search them.
If you are able to trace your family to a specific townland, perhaps from church records, I may be able to help you search a bit further.
If any of the Watt family remained in Donaghmore and were farmers then you would expect them to be listed in the tithe applotment records. There’s just one Watt in the 1826 records for Donaghmore. He was a James Watt and farmed in Tullyarran:
https://cotyroneireland.com/tithe/donaghmore.html
Something to bear in mind if you can link your family to that townland. In Griffiths Valuation for 1860 there were 3 Watt households (likely connected as they were beside each other). Mary, Jane & James. The 2 ladies were probably widows.
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
One Watt farming there in the 1901 census. He was Presbyterian.
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Crossdernot/Tullyaran/1735685/
I agree that your ancestors may well have originated in Scotland. The surname, location and likely denomination all point to them arriving in the 1600s as part of the Plantation.
Can’t help you with your clans. It’s not something that most folk in Ireland take much interest in. And most of the Scots settlers who came to Ireland came from Lowland Scotland where the clan system didn’t prevail to the same extent as in the Highlands.
Watch out with tartans. Most of the more modern tartans are Victorian inventions created for wealthy visitors. Queen Victoria acquired Balmoral Castle and suddenly Scotland became very fashionable for wealthy English visitors. They liked to dress up in “traditional style” and there was a commensurate growth in tartans and highland paraphernalia. The Scots did wear plaid but I don’t think there were really many genuine tartans in the old days. At the battle of Culloden (1746) the various clans used crests for identification. Clearly if they wore tartan at all, it didn’t identify them. If you are interested in the background, a good read is Hugh Trevor-Roper’s book: “The invention of Scotland” especially the chapter entitled “The sartorial myth.” It was apparently an Englishman named Rawlinson who invented the modern kilt because his Scots workers were having difficulty felling timber in their plaid, which was too cumbersome.
Finally, Sally is a common diminutive for Sarah, both in Ireland and in Scotland
