Dublin Airport says it doesn't have exact passenger numbers on a daily basis.
It comes after big queues were seen at the airport, and outside the terminal, on Sunday morning.
The airport is offering refund vouchers to passengers who missed their flights.
Graeme McQueen, media relations manager at Dublin Airport, told Newstalk Breakfast they go on information fed from airlines.
"We have to adapt on the day if things change - staff absenteeism, higher passenger numbers than we were expecting.
"That's commercially sensitive information from airlines - we take feeds from the airlines, we have an indication as to the number of passengers that are coming in.
"We have an indication of how many, but we don't know the exact numbers.
"So numbers could be slightly higher on the day, but we plan for that typically.
"We accept fully what happened yesterday was not good enough.
"Since the end of March... we have been trying to stay ahead of the very fast rise in passenger numbers.
"Yesterday they caught up with them, but if you look at the months before that we've 98% of passengers through in less than 45 minutes.
"Good progress has been made - I accept fully that we tripped up yesterday - we will be working very hard in the coming days to make sure that that trip up doesn't happen again in the future".
'It wasn't good enough'
He says very early presentations by passengers on Sunday set the airport back "for the day - and it's very hard to recover".
While the advice to turn up two and a half hours before a short-haul flight, and three and a half hours before a long-haul flight, still stands.
"We will look at that again this week based on what happened at the weekend - if we need to change that we will do so, and communicate that in advance with passengers".
Mr McQueen also apologised to those affected.
"We're going through a process yesterday, today - we'll look at it in the coming days as to exactly what went wrong.
"I think we saw scenes at Dublin Airport yesterday that disappoint us.
"We're really, really sorry to any passenger who was affected by it.
"I was out there myself for the day, I saw the scenes first-hand - it wasn't good enough, as an airport we have to do better".
And he says they expect similar numbers for the upcoming long weekend.
"We've got a very busy June bank holiday weekend coming up, we'll have 100,000 passengers per day.
"There'll be similar numbers to what we had yesterday - 50,000 departing, 50,000 arrivals as well yesterday.
"That's what we'll see again next weekend: we're working today, tomorrow... and the coming days to make sure that we've got staffing where it needs to be, to make sure we don't see any kind of problems anywhere near what we saw yesterday".
