The HSE Live twitter feed is updating on queuing times in different centres each day. So it might be good to check there. My sister got hers 2 weeks ago, they turned away all walk ins under 60 and had separate lines for those with appointments.
My husband went last week, asked if they were doing walk in vaccines, they said yes, but there was only one queue, and when he got to the booth the vaccinator was surprised he was there for a walk in booster because they were not doing them that day. He still got it but realised his original query was mis-leading, he should have asked if they were doing walk in boosters.
But it sounds as if from last Friday they have ramped up walk in booster shots at all clinics. So peoples experience from last week or the week before is no longer valid. This week is about giving as much Moderna as possible. But it may be faster in middle of the day. There seems to be delays at start and end of day mainly.