As others have indicated, Ryanair is like a 'flying bus'. i.e. when you board a local bus you don't have reserved seats; you just look for what spaces are available. Exactly the same applies on Ryanair.
Ryanair does offer passes which enable you to be in the first group to board. (If you've not bought them online there may well be someone walking up and down the queue at the airport, offering them for sale). If you're travelling with young children (where, of course, it's essential that your party are all seated together) it MIGHT be worth purchasing such passes but, as long as you're among the first to join the 'normal' queue (by getting to the airport early) it's probably still not worth it.
If I'm to the only passenger sitting in a row of three seats, and I see a family group looking for seating together, I always offer to move. (As, I'm pleased to note, do many other people).
Chris