When you don't know where to begin looking?

Post us the full details. When was your ancestor born? Where and who was he living with in the 1841 census?

If he was Presbyterian than he’s likely to have come from the counties of Ulster as that’s where most Presbyterians were found. (200,000 Scots moved to Ireland in the 1600s bringing Presbyterianism with them. Most settled in Ulster. Presbyterianism is often a clear sign of Scottish ancestry). By the 1800s the Scots settlers were often moving back to Scotland as the employment prospects were better than in Ireland.  Ireland lacks natural resources. No coal, oil, iron ore etc, and so apart from a modest amount of shipbuilding in Belfast and the Belfast linen mills (which mostly only employed women), it did not really get the industrial revolution that benefited England and Scotland where mills, steelworks, ship building, coal mining and all their support industries were major employers creating hundreds of thousands of new jobs. Much better paid than subsistence farming or weaving. Added to that you had the effects of a massive population explosion in Ireland – up from 3 million in 1750 to 8 million in 1841 (no-one is really sure of the reasons why but reduced neo-natal deaths seem to be a factor) and the famine. So some push factors and some pull factors saw huge numbers of people leave Ireland. Something like 8 million people emigrated from Ireland between 1801 & 1921.

https://www.irish-genealogy-toolkit.com/emigration-Ireland-19th-century.html

If you look at the Scottish censuses for the Glasgow area in the late 1800s, you will see that about every fifth person recorded there was born in Ireland. Scotland was a particularly popular place to go to work because it was easy and very cheap to get to. Several sailings every day from Belfast, plus regular sailings from Portrush, Ballycastle and Londonderry, not to mention Dublin. The shipping companies main business was cargo and the passengers were just top-up revenue. Competition was fierce and passenger fares very low.

Moving on to another place – in this case New Zealand – was also very common. It’s called stepped migration.

I did a search on rootsireland for a marriage in Ireland between Peter Quin & Catherine Gorman but did not find one. Likewise no trace of a baptism for daughter Sarah. It’ll likely be pretty hard to trace the family’s origins. Check all the Scottish censuses. Usually place of birth is just “Ireland” but occasionally they put something more detailed. If Peter Quin died in Scotland, 1855 onwards, his death certificate should give you his parents names and occupations. Also whether they were alive or dead at that date.