Haaaaaaaa! In your faeces/faces non believers! Straight from the Ryan Air website

"It is not necessary from a safety perspective to deflate typical tyres found on bikes and wheelchairs for carriage in the hold. However, to eliminate the small risk of them being damaged by bursting, you may wish to deflate the tyres. Understandably this decision may be influenced by how easily the tyres could be inflated upon arrival."