Selections from David McKittrick and David McVea's "Making sense of The Troubles" (2000), apologies for the lack of variety, it's the only text I have at hand.

On segregation:

While there had always been a large measure of segregation between Protestants and Catholics, particularly in Belfast, the Stormont system helped ensure that such patterns would continue. The two communities mixed in some fields, but in their housing, education, and very often in their employment they kept apart. The situation was summed up in 1971 by a Catholic observer who said:

"If there is one thing which I have learned in my 30-40 odd years as a community social worker it is this: that, broadly speaking, two communities have lived side by side in Northern Ireland without really knowing each other, or without making any real honest, sincere and conscious effort to bridge the communications gap."

Politics atrophied in both communities. Many Catholics simply did not vote or voted only rarely, succumbing to apathy brought on by the realisation that the Unionist voting machine was invincible. In contrast to the highly effective communication skills which it would develop during The Troubles, the Catholic community was able to attract little attention or sympathy from outsiders. (p.18)

On preferential treatment and jobs:

Although it seemed for a fleeting moment that the new police force, the Royal Ulster Constabulary (RUC), might become religiously integrated, it remained throughout its history more than 90 per cent Protestant. The police had no real operational independence, responding directly to directions from ministers, with senior police officers sometimes attending cabinet meetings. The political, legal and policing worlds were thus inextricably linked: one community governed, judged and policed the other. These security forces, once pointedly described as the armed wing of Unionism, not only maintained law and order, but also provided jobs or Protestants.

This was one of many points of Protestant advantage in the field of employment. The civil service was predominantly Protestant, with perhaps 10 per cent Catholic representation in its lower reaches. A 1943 survey established that there were no Catholics in the 55 most senior jobs, with only 37 Catholics in the 600 middle ranking posts. The picture was similar in most local authorities and other parts of the public sector. There were no Catholics among the cabinet (there wouldn't be one until the early 70s), the senior staff in the Stormont commons, the top ranks of the RUC, the Civil Service commission and other important public bodies. (p.11-12)

I think you may be mixing up pre- and post-partition history a bit, tbh:

From the 17th until the 19th (arguably until 1920, in fact, although none of any consequence still existed by then) Ireland was ruled under a series of laws, acts and statutes collectively referred to as the "Penal Laws", which essentially limited the public and political freedoms of Catholics and "dissenters". This latter group included any non-catholic church not in communion with Westminister: today we would regard them as also being Protestant, but at the time that term applied pretty much exclusively to the Church of Ireland.

These laws included a number of provisions, most famously against the holding of public office by Catholic individuals, and by the 19th century began to apply more or less only to Catholics. This was because there was a deliberate attempt to drive a wedge between the prosperous and politically motivated Northern Presbyterians and the Catholic majority after the 1798 rebellion, that included the British lessening restrictions against "dissenters" as well as emphasising sectarian attacks against Presbyterians during the rising in propaganda.

You may be confusing this largely de jure discrimination of Catholics and Presbyterians with the more de facto discrimination that occurred in NI after partition in 1920/21: another commenter has covered that quite well, so I'd recommend you read their response if you haven't already. Sufficed to say, Catholics in NI often weren't discriminated against in law (although that did certainly happen too), but it was rather more of an informally recognised system of discrimination, than a legal delineation of rights to different groups.