What do you mean by a "regular sized rucksack" what is regular-sized? If you are talking about school-bag size that should be fine, anything bigger is usually too large.
Thanks for the replies.
Yes, about a schoolbag sized is right and it wont be overfilled or heavy.
Regulations would have to be pretty darned tight these days not to permit it, but then you just never know, do you?
Hi, here are the measuremants and weight for cabin baggage. From the Aer Lingus website.Permitted one piece less than 6KGS/ 13 lbs and measuring 22 inches by 18 ins by 10. 56cms/45cms/25. The rucksack should be fine as I travelled two weeks ago with same and observed other people bringing on as hand luggage what can only be described as small suitcases but well above the measurements allowed.