So, last week I turn up at DUB with my online-printed boarding pass for an 
FR flight to England.

All is fine until boarding at the gate, when I'm told that I can't travel 
with an online-printed boarding pass and a UK *photo* driving license.

As I had done just this very thing not 2 weeks previous on a route from SNN, 
I protested - along with another passenger who had travelled this way half a 
dozen times, ex-Dublin, to the same airport.

Ryanair - get this - seemed to be basing the decision not to let us travel 
on the fact that when you check in online for FR, you have to provide your 
"Passport or ID Card Number" [says the field on the web page you complete 
when checking in].

Of course, knowing that the UK and Ireland are Common Travel Areas and that 
a UK driving license is, in other circumstances, perfectly acceptable ID for 
a flight between or within those countries, one *assumes* [d'oh!] that it 
would be fine for online check-in as well, and I had previously provided 
this and travelled without difficulty.

Apparently not.

Their website confirms that, if you are a UK or Irish citizen, that a photo 
driving license is OK for travel between these points [see 
http://www.ryanair.com/site/EN/faqs.php?sect=pid&quest=photoid].  Compare 
and contrast with the details on online check-in at 
http://www.ryanair.com/site/EN/faqs.php?sect=web&quest=documents

Ryanair denied both of us travel solely and exclusively on the argument that 
because the paperwork says "Passport or ID Card Number" - rather than "ID 
reference number" or similar - that we couldn't travel.

I explicitly put the question to the station manager: "If I had checked in 
at a desk in the airport, rather than online, with the same ID, could I have 
travelled?" and was told yes, I could have.  Of course, by boarding time, 
checkin for that flight was closed [natch].

So, on top of all the requirements you need to check about what form of ID 
is acceptable, you now also need to factor in your method of check-in (!). 
In summary [sorry for rambling but you can imagine how cross I was!]:

UK citizen + driving license + airport checkin for UK/Ireland/UK flight = OK
UK citizen + driving license + online  checkin for UK/Ireland/UK flight = 
*NOT* OK

This must score at least 7 out of 10 in the "bloody stupid and arbitrary 
rule changes" index.

I ended up forking out for an alternate flight on bmi, plus extra taxis and 
hotels.

B*stards.

T