I wonder why Catholics in Ulster got paid less, if hired at all than Protestants?
I think that one is mostly or entirely myth. There were a few companies that mostly hired from one community. Sometimes it was discrimination, sometimes because people known to people working were first choice and sometimes location. In industrial areas: West of the Bann, the "protestants" concentrated in Waterside of Derry and East of the Bann, Catholics mostly in West Belfast.
In a particular company doing same job, the "Catholics" and "Protestants" (detestable way to categorise N.I. People) got same pay, except very rare situations.
The border was artificially drawn to ensure a Unionist Majority, leaving out 3 counties of Ulster. Let's not however go backwards but forwards, that only benefits extremist bigots such as DUP and SF.
I wonder why black people got paid less money than white people in the American South
I wonder why despite all the claims of exporting Democracy, large parts of USA didn't have it till 1960s?
It's easy to make simplistic judgements.
assigning the jobs per se so much that one religion simply didn't work in certain fields of work
Absolutely. Also mobility (to move house) and travel distances very low. Actually physically communities are mostly MORE segregated now than before 1969. I think the big secondary school (where my "Catholic" friends went*) is closed due to increased segregation. What I said above about locations of communities and companies.
So called "Protestants" that go to UNI now tend to go to mainland UK and many don't return.
At primary school age, the so called "Catholics" may be in majority now, though that shift has slowed down.
[* Unusually I grew up with about equal numbers of friends in both so called "Communities"]