Bus drivers reported several instances of double-deckers reaching capacity between the first and third stops yesterday as the 25pc capacity rule kicked-in on the first day of the second lockdown.
Routes in Swords, Skerries, Rush, Ongar, Damastown, Celbridge and Lucan in Dublin appeared to be particularly badly hit.
Many people who don't live close to their terminus faced the risk of being left at their stop during peak periods.
Drivers said the 25pc capacity was operationally realistic during the lockdown in March and April, but it is leaving many people stranded at bus stops this time because schools and more workplaces are allowed open.
"Our members and frontline colleagues have experienced difficulties in trying to accommodate passengers on services across a number of bus routes as the 'new' Government 25pc capacity rules come in effect," said Dermot O'Leary of the National Bus and Rail Union (NBRU).
"There has already been several pinch points and locations where passengers have tried to gain access in large numbers to Dublin Bus services.
"Some drivers are describing it as 'mayhem', as excessive numbers of commuters try to board buses across a range of routes," he said.
"Some Bus Éireann services have also experienced capacity constraints where demand has outstripped the Level 5 reduced capacity."
There were also reports of people bunching at doors to buses and social distancing breaking down as a result, but compliance with wearing face masks was reported to be good.
"Thankfully, thus far at least, no confrontation or conflict has been reported, though we are extremely conscious that frustration can lead to such a scenario.
"We cannot blame passengers, striving as they are, to get to their place of work, at a time which suits their particular needs, as opposed to when services may, or can be scheduled," Mr O'Leary explained.
"The NBRU has, from the commencement of the coronavirus crisis, been calling for a plan to be put in place to both accommodate those that are required to travel for essential purposes and also to be given a clear outline of how capacity restrictions were to be policed," he said.
"In two separate submissions to the Oireachtas Special Committee on Covid-19 Response, the NBRU called for the staggering of school, work and retail start/finish, opening and closing times, to manage public transport demand," he added.
"It is neither fair nor sustainable that you would have those drivers that operate the service and those that use the service, in conflict with each other.
"I have now written to the CEO's of Dublin Bus, Bus Éireann and Iarnród Éireann to ascertain their plans for additional services and how they propose to police capacity issues," he explained.
"It is not the job of a bus driver to police public health guidelines.
"The Government needs to consider wider societal measures during 'Lockdown Two' so as to manage public transport demand."