From what I can see on Google, there's different sizes available of the 24/7 rucksack. The 24/7 30 is too big to be a personal item on Ryanair*, but if you've paid extra to have a larger bag in the cabin then it's fine. The 24/7 20 is very slightly too big, but in all honestly will be absolutely fine, and as long as it isn't packed solid it'll probably fit into the sizer too.

*my experience is that any soft bag will be accepted as long as it isn't absolutely huge, but obviously you can't rely on this. The gate agents mainly look for people bringing hand luggage cases without paying for priority boarding.