Outside of an emergency evacuation or similarly dire situation, I cannot think of any reason why a civil authority would ban a person from purchasing multiple seats. After all, it is not uncommon for people to purchase an additional seat to carry items like musical instruments or pets, or because they do not fit into a single seat.
In consideration of this, Ryanair not only explicitly permits such a purchase, but provides a page of instructions on purchasing an additional seat on its website; while it is listed under purchasing for a "large person," the wording includes extra seats purchased for carrying items:
Note highlighted section
To book an extra seat for comfort go online and select 2 seats the first seat should be booked in the actual passenger's name and the extra seat should be entered as COMFORT SEAT as the Last Name and EXTRA as the first Name. The name EXTRA COMFORT SEAT will then be displayed on the reservation and online boarding pass.
Allocated seating in emergency rows 1,16 and 17 may not be purchased if you have purchased an extra seat for item/comfort.
Please note that all passengers purchasing an extra seat can use online check-in. The same travel document details must be entered for both seats when checking in online. Online check-in is available from 30 days prior up to 2 hours before the scheduled flight departure times if you have purchased allocated seats. If you choose not to purchase an allocated seat check in will only be available from 7 days up to 2 hours prior to each scheduled flight departure. There is no cabin or checked baggage allowance associated with the purchase of an extra seat.
Customers who have booked an extra seat for comfort should contact our call centre to ensure that they are allocated seats together.
** Spanish Resident and Large Family Discount is not applicable on EXTRA seats**