Regarding the record of William’s departure for Nova Scotia c 1830, in my experience there are few passenger records for journeys to Canada prior to 1850. There was no requirement to compile them. A few shipping agents records have survived but that’s about it. What there is, is mostly on the Mellon site:
http://www.qub.ac.uk/cms/
Tracing Wm S & Jane King’s antecedents will be tricky. There are no church records for that area prior to 1819. So neither William’s baptism nor his parents marriage is likely to be in any surviving records.
I note that the Rev Sommerville was a Covenanter Minister. So possibly his parents were also Covenanters. There is mention of Covenanter meeting in the Aughnavallog area c1770 on the Rosdavies site.
http://freepages.genealogy.rootsweb.ancestry.com/%7Erosdavies/PLACENAMES/A.htm
The Reformed Presbyterians (Covenanters) in Ireland don’t really have any records for the 1700s. The first ever RP Minister in Ireland was the Rev William Martin who was ordained in Ireland in 1757. His “presbytery” was all of Counties Down, Antrim, Londonderry & Donegal. He lived near Kellswater in Co Antrim but preached all over Ulster. Initially, he had no church and preached in barns or the open air. He kept no records in Ireland (or none that have survived anyway). The first RP church was built at Kellswater. A second Minister, Matthew Lynn, was ordained in 1763 and took over the Bannside part of Rev Martin’s presbytery (ie Cos Derry, Tyrone & Donegal). By 1772 4 more Ministers had been ordained. A grand total of 6.
However by 1779 the presbytery collapsed because 2 Ministers had emigrated to the USA and 2 others had died. It was re-established in 1792 with 6 Ministers. There were still only a handful of churches: Kellswater, Letterkenny, Conlig, Dervock & Kilraughts being ones that I know of. Around 1793 the Ministers for Kilraughts and Kellswater both had to go into exile for preaching treasonous sermons. Back to 4 Ministers, all covering huge geographic areas.
So the picture that emerges for the 1700s, is one of an embryonic organization, with few church buildings, a handful of Ministers (some of whom died or left at short notice), and even fewer records.
In short, if your ancestors were Reformed Presbyterians who lived in Ireland in the 1700s, I don’t think there are any church records you can usefully access.
I had a look in the Rev Hutchinson’s history of the Reformed Presbyterian Church (1893) and there’s no mention of Rev Wm Sommerville serving in Scotland, so if he was a Covenanter Minister by the time he arrived in Canada, he may have been ordained in Ireland. You might want to contact their historical society to see if they have any information on him:
http://rpc.org