I fly quite regularly into Dublin airport and yes sometimes there is a queue, but even if the queue seems massive it only takes 10-15 mins max to get through, it just feels a lot longer. Dublin airport is really not the worst offender. Getting out at Amsterdam usually takes a lot longer. If there is a queue in Dublin there will be at least 10 channels open, in Amsterdam I found many times only 4 or 5 channels open.
I do agree with the point though on the internal flights passengers having to go through passport control, that is just such a silly design flaw.
When ive seen this happen its been late and there are only 2 - 3 officers at the desks. The problem is that they allow everyone through any line no matter where they are from. So this slows everyone up.
I can assure you that when things are bad that it takes a hell of a lot longer than 15 minutes to get through and there are only a few gates open too.