Hello Alison,
I had a quick look in the online church records at
http://www.irishgenealogy.ie but alas, matching records are not easily forthcoming.
Some possibilities:
-- The indexing is hideous, and Sweetnam may be Swetnam or Sweete or maybe even Sweeny if somebody is careless. Connell might be indexed as Connelly. I have seen other indexing horrors but I'm assuming the names are not too mangled.
-- Some Skibbereen records don't start until the 1830's (Castlehaven, which has no marriage records); the 1840's (Kilmacabea). Some Skibbereen parish records are horribly faded, which calls any indexing on them into question. Somebody needs to extract the images and try to improve the contrast on them.
May I ask what records do you have that indicate they were in Skibbereen. If the record implies they were from the Poor Law Union of Skibbereen (rather than the market town) then that was a considerably smaller land area at the end of the famine, when Skull and Clonakilty PLUs were carved out of Skibbereen. It is probably natural when you are far from home and somebody asks "where are you from" to give the name of the nearest large town.
I had a quick look at the Tithe Applotment records I have copies of. There are a few William Connells in some of the local parishes. Skull parish shows a William Connell two lots away from Austin, Mathew and Samuel Swetnam. That's in or near a townland called Letter. In Aghadown parish, in the townland of Knocknaraha are a William Connell and a Luke Swetnam next to each other.
I just gave a presentation today on using autosomal DNA testing as a family history research tool to land yourself in the general ballpark of the right research area. You may want to add that to your research arsenal.
An additional question: Was this family Roman Catholic?