I’m a recent ND grad and an atheist, so here are my experiences.
When I got to ND, I was a casual Catholic, but the increased exposure to the religion during my freshman year kind of turned me off and I had stopped believing by the start of my sophomore year. Most of my friends were Catholic, either nominally or moderately so, but several were atheists and I also knew some “extreme” Catholics, though none were personal friends of mine.
You have to live in a dorm your freshman year (and will probably live in the same dorm through junior year), and all the dorms have their own chapel (well, two particular dorms are conjoined and share a chapel). The rectors who run the dorms are often priests, but not all of them are. The religion is certainly there for anyone who wants it, but you’ll never be forced into going to Mass, and I can say that as someone who lived in a dorm whose rector was a fairly conservative priest, most people didn’t go to Mass regularly and no one cared whatsoever.
The academic curriculum is completely divorced from any religious bent whatsoever. The Catholic Church accepts the Big Bang, evolution, and all that, but it’s more that the academic departments (made up of faculty that have no religious test and who are chosen by the same standards as at secular schools) make the hiring decisions and set the curriculum, so religion doesn’t enter into it at all. I learned about all the horrible things done by the Church in my history classes, with no special justification beyond the historical context in which everything was placed. I’d be surprised if most of the professors were Catholic.
The only exception here is that everyone is required to take two Theology courses (one intro and one elective). These aren’t taught in a “Catholicism is right” kind of way, but the intro class is definitely a “this is what Catholics believe” class. My elective was a course on atheism. There are courses on Islam, Buddhism, Eastern Christianity, comparative religions, the development of Catholicism, liberation theology, and all sorts of things. I didn’t particularly want to take Theology courses, but now I can discuss religion in an informed manner, which can be helpful.
There is basically one big way that the school’s religion will interfere with your life, and that’s with the rules for students living on campus. The dorms are single-gender and you’re not allowed in an opposite sex dorm (except for designated 24-hour spaces) after midnight on weeknights or 2am on weekend nights. It’s also against the rules to have premarital sex, though if you keep your door shut, it’s very difficult to enforce that.
Here’s one student’s perspective on this from an article in the student newspaper last year:
http://ndsmcobserver.com/2015/04/catholic-school-vicious-roman-rule-2/
The writer was a little… harsh here, to be sure, but the sentiment is one shared by many.
On the whole, I wouldn’t change my Notre Dame experience for anything. However, the dorm rules could be frustrating and are basically the one way in which the school’s religion will interfere with your life even if you choose not to practice it.
Please feel free to ask me any questions.