advice on unknown euro airlines

these airls are not at all ''unknown'' they are VERY well known here in Europe!
Both EZ and AB are no-frills lowcost operations, that do NOT do any through-interline check in: not even on their own connections (on EZ, AB does)
you overlooked the very essential: Cro is STAR alliance and thus teams up with LH, OS and TAP; both OS+TP fly also VIE-LIS and the far better connection is via Munchen. this will NOT at all be cheap for an ignorantly booked single.
VIE is a fairly small and efficient airport. BUT; as you must checkin a new you MUST pass passp control there (anyway, as for to go Port. is schengen). you must hence check the latest check-in for AB: I guess 60 mins!
In Paris you must double check: 2 main airpts, and I think they use the different ones.
BTW: these Qs are far better posted and answered by knowledgeable people on flyertalk.com

thanks so much for your info, bmta. Yes, we are Americans who will be in europe...and have been many times....but not flown on any of these airlines.

you have clickair - Dubrovnik - barcelona for 50 euro with tax

You can check out readers' reviews of airlines at www.airlinequality.com.
Air Berlin may be one of the best airlines in Europe - even though it's a budget airline (not really cheap on all routes though) it scores better than many flag carriers.
You'll find a great overview of budget airline routes at www.whichbudget.com.

ther very cheapest is, if you have them, to spend UAL miles,as as I said all on STAR is likely the best.
point is, that mid-apr is NOT yet the best time for flites to DUB, as most of them are very seasonal and non-daily (aiming at what used to be former ''charter'' for holidaymakers). I think CLick will only start later.
use skyscanner.net for best overview of low-budget fares. also check germanwings, via their hub in Koeln=DE. but there are dozens of those sites listing all these lowcost; some like i think mobissimo even give connections. again, you may be hampered by the lack of flites ex DUB at that time.
and AB is NOT at all the best airl of EUR (thats likely swiss or LH), but is in the lowcost segment! Ive flown on all, and yes AB has a tiny bit left of ''normal'' airls: free food (=a sandwich), and newspprs (german only).