Yes you can use your mt bike as a beginner. The biggest disadvantage is that it will be considerably heavier than a cross bike, so places where you need to get off the bike and run with it (often uphill.... I think maybe even up steps at PIR) will be more difficult and the geometry of a mt bike may make it difficult or impossible to "shoulder" it (carry your bike on your shoulder). But you won't be alone - there will be other people using mt bikes.
Is it hard... I don't know how to answer that. The races are pretty physically intense and you'll do best if you have good bike handling skills. If you ride your mt bike off road you should already have an advantage over someone who is coming into it from road - depending on the course. A technical course with a lot of descending and turning will often favor a mt biker, while a less technical course with more flats may favor roadies.
Tips - take a class. Learning to mount and dismount properly is important. You can lose a lot of time if you aren't doing that well. There are techniques for doing barriers and taking corners as well too. If they offer classes or weeknight practices - go and learn.