As an example, Alaska Airlines in the US won't allocate you a seat till the last second if you fly in the cheapest class, however they (generally) will always try to seat people together if they're traveling on the same reservation.
I have a ticket with Ryanair from Pisa to Prague for two adults. If I don't pay for a seat, is Ryanair likely to seat us apart? I've never flown with them before with another person so don't have personal experience.
We recommend that you reserve your seat when booking or when checking in to guarantee a seat beside your travel companions. If you choose not to reserve a seat, then a seat will be randomly allocated to you free of charge when checking in, but it is unlikely it will be beside your travel companions.
(from Ryanair Seat Policy)
Ryanair has also had to deny that they deliberately separate such seats, so your chances are at most as good as random.
In general, Ryanair is a low-cost airline whose business model heavily relies on cheap base prices and a large amount of additional paid conveniences, of which seat selection is one, so I wouldn't expect to get it for free - certainly not as a matter of course.
Unlike other airlines, when flying with RyanAir assume you'll be sitting apart from the person you fly with unless you pay for seats. RyanAir deny they deliberately separate people but they are clearly lying about this.
Since there's been a lot of discussion about the side point of whether RyanAir's policy of separating people on their flights can count as "deliberate". There is a technical sense in which you could argue you that it is not - what they seem to be doing is simply allocating each customer a seat at random rather than what (almost?) every other airline does and trying to place customers who booked together next to each other when possible whilst doing this allocation. However, in my opinion, the choice of how to assign seats is clearly a deliberate act and thus RyanAir's decision to go against that counts as them deliberately separating people.
Back in 2018, a CAA survey found that RyanAir were already separating passengers over twice as often (35% of passengers) as was normal for other airlines. However, although I can't prove it, I am quite certain that they changed their method of seating after this time to make it much worse. Certainly it was after this time that everyone I know who flew RyanAir found themselves being separated, and when I experienced the same.
Personally, for us, this was the point that we decided RyanAir had gone too far and decided to never fly with them again. It's one thing to charge for actual extras that better airlines include for free. It is quite another to deliberately make the experience worse unless you pay them.
They do seat you deliberately apart to try to sell you seats. They allocate seats from back and front of the airplane, not by the sequence like other airlines. So every other seat goes from the front and usually mid seat and the next from the back.
However, there is an easy to trick to sit together in premium seats on Ryanair flights for free.
