Actually, it depends on the airport. Stansted is very very strict and I would not recommend you to bring an extra bag.
As for the other routes: Go ahed. I've flown 17 sectors in the last three months on Ryanair, all the time with a 57x42x18cm trolley that always weighed more than 10kgs and on ten flights I had an additional bag with me. I was never even looked at, dare even questioned about the size or weight.
And I have to correct Raffles as well. There is no such thing as a commission on charges for Ryanair staff.