I use Ryanair regularly, nothing wrong with them provided you stick to the regulations.
As to Dublin~Edinburgh. if you want a cheap option without having to worry about baggage restrictions you can coach and sail between the two locations. does take a little longer though, http://www.stenaline.ie/ferries-to-britain/coach-sail

The last time I flew with Ryanair they were fine - no frills, but quietly efficient. You do realise though that your 65l backpack is very unlikely to fall within their cabin baggage allowance, and you will probably be paying to check your bags?

I spent a decade traveling with a backpack, the last few years I've traveled with a wheeled case instead. In both instances I also took a day pack, which was my carry-on for flights and was big enough to hold all my electronics plus pharma and a change of undies and top. I always check the bigger bag (toiletries, corkscrew, nail file etc., plus less hassle). In general I prefer the wheeled bag, but not when doing a lot of train travel.

I would certainly opt for the smaller bag - I think my backpack was 45L - but as noted above what counts is the weight. Mine usually weighed in for flights at 10-11 kg.

I would cut back on the clothes (three or four shirts, one dress, four underwear) but up the socks as they take longest to dry (I hand wash all my clothes, usually every other night - don't take anything that won't drip dry).

BOTH of you should wear a money belt (I use this one, with the pouch at the back: http://www.backpacktravelstore.com/L...p/grlkwmbb.htm )

You should definitely check both your bank and credit card fees, and consider not just a Credit Union account, but chip and signature/pin credit card from Pentagon Federal. (Do a search here on chip and pin for more info)

For my packing list (dated on electronics, but otherwise what I'm still using) start here (it's three posts!): https://mytimetotravel.wordpress.com...take-part-one/

Getting the best rate for checked-in bags with low-cost carriers like Ryanair means booking the bag on line in advance. You'll normally end up paying a much higher rate if you wait until you arrive at the airport! As for saving money, in my experience (frequent traveller, London based) - if you book long enough in advance it is often as cheap flying with regular airlines such as Are Lingus etc. as the low-cost operators.

On longer trips I tend to take a couple of old T-shirts to wear that I can throw away during the trip once they get past the point of no return, to be replaced with new ones bought along the way. And I never take shorts, but wear zip off hiking pants - saves another item in the bag.

The first time I travelled with a full sized backpack I was completely unprepared for how heavy and awkward it really became after a few kms of walking round in circles looking for youth hostels....
I then bought a full sized wheeled suitcase, what a disaster. Never considered how I was going to hurl that onto an overhead shelf on a train....
Subsequent trips led to smaller bags.

I don't mind not buying souvenirs and handwashing regularly, so I use a smallish back pack that also has wheels, it would be acceptable carry on, but we check ours in. It weighs about 9kg once packed. The backpack option is good for cobblestones or stairs, but mostly I wheel it. Mine has plastic wheels, I think rubber wheels would be more robust and less noisy.

I know many here aren't Rick Steeves fans, but check out his web site on packing and luggage. It might give you some other ideas.

When it comes to backpacks the most important thing is the hip belt. It's supposed to take most of the weight, and if it doesn't the weight goes on your shoulders and gets to be a real pain real fast.