Your error with Ryanair shows up in your sentence which reads "so i went to checkin and they charged me 30 euro".
You SHOULDN'T have gone to the baggage check-in desk as they apply a charge to anything that they put into the hold. (They have to assume that it's additional to any hand baggage you might want to take onto the plane).
ALL passengers with Ryanair can take TWO bags TO THE GATE. As long as they both meet the relevant dimensions (i.e. no more than 35 x 20 x 20cm for the small one and no more than 55 x 40 x20cm for the large one, which must also weigh no more than 10kg) there will be NO CHARGE.
However only passengers who've paid for 'Priority' can take the larger bag on board. Anyone else will have their larger bag placed into the hold (meaning that they'll have to wait for it in the baggage reclaim area upon arrival).
So (unless you've got more baggage than the two items I've mentioned above) you shouldn't go anywhere near the baggage check-in desk on your next Ryanair flight. You should simply take your two bags to the gate.
Those two bags (with a maximum weight of 10kg for the larger one, and NOT the 15kg you referred to in your post) are all that you can take for free. If you need to take additional bags (which you'll have to take to the baggage check-in point) you'll be charged £25 (or €25) per bag if you book them in online or £40 (€40) if you simply turn up at the airport with them. You're allowed a maximum of 3 such bags, with a maximum weight of 20kg each.