The selection of the 90 items is a bit random, depending on the preference of the local staff. Here are some of the variations I have seen:
1. Selection of people as they pass through the boarding gate. Certain types of passenger appear to be targeted, e.g. young couples and groups who won't argue and those with paper boarding passes.
2. Offering it to passengers in the queue like an extra service you're getting for free.
3. Offering it to passengers at the bag drop and/or ID check desk as an extra service you're getting for free (but you still have to carry it through to the gate).
4. Forcing it on random people in the waiting area (e.g. Warsaw Modlin). The key in these cases is to hold back, so they don't know which flight you're on.

I'm confident they let more than 90 bags on, as many people (e.g. myself) will happily put a rucksack under the seat on a short flight.

There's a few arguments to keeping your bag, e.g. there are items which the Ryanair T&Cs prohibit from travelling in the hold. This didn't work for another passenger at Modlin. Should the worst happen, the yellow bag tag could easily disappear between being attached and arriving at the plane where you deposit the luggage in cage.....

I've flown 13 Ryanair sectors in the last year and never been asked to check in my carry on. I have been approached in scenario (2) and (3) above, but it has never been forced on me when I declined.

In terms of boarding order:
1. Disabled and priority boarding
2. Everyone else in the order they are in the queue

If you really want to pay for priority boarding, you can do this for each sector separately when you check-in, or through "Manage Your Booking" now.