Current rules as per Ryanair T & C on Cabin Luggage:
CABIN BAGGAGE
Strictly one item of cabin baggage per passenger (excluding infants) weighing up to 10kg with maximum dimensions of 55cm x 40cm x 20cm is permitted. (handbag, briefcase, laptop, shop purchases, camera etc.) must be carried in your 1 permitted piece of cabin baggage.
Extra/oversized cabin baggage will be refused at the boarding gate, or where available, placed in the hold of the aircraft for a fee of £50/€50. If you are unsure, check at the Bag Drop desk before going through security.
Smaller musical items such as a guitar, violin or viola which exceed our cabin baggage dimensions may be carried in the cabin if a seat for it has been reserved and the appropriate fare paid. There is no checked or cabin baggage allowance associated with the purchase of an extra seat. To book an extra seat for an item the word "ITEM SEAT" must be entered as the surname and "EXTRA" must be entered at the first name. EXTRA ITEM SEAT will then be displayed on the reservation and online boarding pass. The accompanying passenger's travel document details must be entered during the online check- in process. Spanish Resident and Large Family Discount is not applicable on EXTRA seats.
You must also fully comply with Article 8 of our Terms & Conditions of Carriage (click here for details).
Last time I witnessed a Ryanair flight boarding, the dispatch crew, physically checked ALL hand lugage at the departure gate, and immediately imposed a charge on ALL "oversize" hand luggage.