Serious concerns have been raised about how Irish Rail measures the punctuality of its suburban train services, after a "snapshot" survey suggested that almost two-thirds of its trains from Belfast to Dublin fail to meet international reliability standards.

By comparison, the company's most recently available punctuality statistics claim that a far smaller percentage – just over one in ten of its trains on the route– are late.

But critics of the company say this is because Irish Rail's own measurement system counts suburban trains that arrive at their final destination within ten minutes of the schedule as being on time, instead of the five-minute rule commonly used in Britain and elsewhere.

The survey, conducted by the lobby group Rail Users Ireland (RUI) between 17 and 21 August, revealed that 24 out of 37 trains (or around 65%), whose arrival times at Connolly station were compared with published Monday to Friday timetables, were more than five minutes late. Twelve of these trains (or around one in three) arrived more than 10 minutes late, and six were more than 15 minutes late, the figures reveal.

By far the least punctual arrival at Connolly was the 8am Belfast to Dublin service which, despite being a important morning rush hour service, regularly arrived between 14 minutes and 21 minutes late.

Times for three other trains on the eight-times-a-day service were not available, partly because of the collapse of a viaduct on the rail line at Malahide during the week in question.

Mark Gleeson of RUI, who used arrival times published on Irish Rail's own website to compile the data, said the figures indicated wider problems on the suburban rail service.

He called on Irish Rail to publish figures for all arrival times on its network and to provide details of how and by whom its own monthly punctuality statistics – which do not provide raw data but provide a simple overall percentage figure for "on time" trains – are audited.

"They're taking 10 minutes as the basis for their punctuality stats on all suburban train services. But in Britain, Northern Ireland and across Europe the standard is five minutes. This standard is now used on the Dart but is not used on suburban services," Gleeson said.

"And anyway, if you are a passenger and you are travelling on a train which is late all the time, then you're late 100% of the time, so even these overall punctuality figures supplied by Irish Rail are irrelevant."

Irish Rail spokesman Barry Kenny dismissed the survey as "extraordinarily incomplete" due to the omission of trains from Dublin to Belfast, each of which he said had arrived on time or within five minutes of being on time on the five days in question.

"The four-week period ending 9 August showed punctuality at 88.9% on this route. Punctuality is measured as on time or within 10 minutes of time, as advised to customers in our passenger charter and punctuality posters. On a quarterly basis, an independent auditor under the direction of the Department of Transport examines our punctuality," he said.

"The year-to-date punctuality on the route is 93%, suggesting that the week (or the incomplete one-way version provided by RUI) was not typical. This was the first period this year when punctuality went below our minimum target of 90%, and we obviously always strive to reach the highest possible punctuality."

However, Gleeson noted that Kenny did not take issue with the accuracy of the arrival times compiled by RUI, and said he had tried unsuccessfully to obtain arrival times for the Dublin to Belfast route.

"We don't have access to the figures from Belfast and would love to get them," he said. "One of the big concerns is that there is no way of validating the numbers they provide to back up their punctuality statistics as they won't give us the raw data. This is unlike other operators such as the Luas, which does provide these figures. If they are confident in their data, then why won't they hand it over?"