Paul,
Joseph & Sarah’s marriage and the baptisms of their children are long before the start of statutory birth & death registration (1864) and statutory marriage registration (1845). To have any chance of tracing them you need to search church baptism marriage and burial records.
You haven’t said what denomination the Rowley & Nixon families were but looking at the 1901 census of the county the vast majority seem to have been Church of Ireland (ie Anglican). So you need to search church of Ireland records, not all of which have survived and not all of which are on-line.
Do you know Joseph Rowley’s occupation? Was he a farmer? If he was then I have a clue for you. There was a Joseph Rowley farming in Skeagh townland in 1832. See the tithe applotment records below.
http://www.irishgenealogyhub.com/fermanagh/tithe-applotment-books/parish-of-aghavea.php
There are no Rowleys listed as living there in Griffiths Valuation c 1860 which is encouraging in a sense as it shows they had left.
Skeagh is in the parish of Aghavea. Aghavea Church of Ireland has records as follows:
Baptisms, 1815-1926; marriages, 1815-1907; burials, 1815-1986; vestry minutes, 1762-1947; list of payers, 1832 and 1834
I had a look through the baptism records for that parish and spotted 4 children that may be to your couple:
Margaret Rowley 13.3.1825, Mary Rolly baptised 2.2.1834 & Robert Rowley 8.1.1837 (Father Joseph Rolly/Rowley and mother Sarah. No maiden name noted). Also Elizabeth Rowley 21.7.1822 mother's name Sally. (Sally is a commont diminutive variant of Sarah). I also see a baptism on 21.6.1817 to Joseph Roaly of Skeagh and Nancy Smith. So that makes me think Joseph was married twice. I may have missed others so worth double checking those records.
https://www.igp-web.com/IGPArchives/ire/fermanagh/churches/aghavea-coi-bap.htm
Though the record is significantly incomplete, I suspect the burial on 31.12.1840 is for your Joseph Rowley:
https://www.igp-web.com/IGPArchives/ire/fermanagh/churches/aghavea-coi-…
I looked for a burial for Nancy Rowley nee Smith 1817 - 1823 but did not find one. She may have been buried elsewhere, or simply not noted in the records.
Skeagh is on the modern Gardener’s Cross Rd a mile or two north of Maguiresbridge.
You wonder where the family sailed from. There are many options. At that time some left from Sligo or Londonderry, but others made their way to Liverpool, Dublin or Belfast and sailed from there. (There were many more sailings from Liverpool than from Ireland itself and the passenger agents often threw in the short passage to Liverpool free as part of the deal). The family possibly/probably paid for the tickets by selling the farm.
I have attached a transcript of a letter (original in PRONI) describing the general situation regarding migration to Canada at that time (1847).