They could have done a very quick clean at Connolly, staff on site to do this. Its not as if the train was starting from an unstaffed station.
There is exteremly limited rubbish bins on the train, if Irish Rail want to operate a trolley service they better be able deal with the waste that appears
You won't see rubbish on the Cork line or Belfast, even with exteremly tight turnaround after delays
In reality a spare train should have been brought in to operate the 16:40 in the first place