Anti-doping officials in the Republic have launched an investigation after urine samples taken from GAA players were lost by couriers on two separate occasions while en route to a World Anti-Doping Agency (WADA) laboratory in Germany.
Sport Ireland has written to the GAA informing it of an investigation into the lost samples.
It has also emerged that there were seven failed attempts by anti-doping officials to test GAA players between June 2016 and May 2017 involving county players from Antrim, Carlow, Kilkenny, Westmeath and Offaly.
In each case the squads were not present for testing despite their training schedules having been agreed months in advance with Sport Ireland.
The investigation into the missing samples was launched in June after urine samples taken from three GAA players with the Westmeath hurling team were forwarded to a WADA-approved laboratory in Cologne for testing. The samples never arrived.
This was the second time that samples had disappeared.
On July 11, 2016, samples taken from three Fermanagh Gaelic football players were lost on their way to Germany by the courier company.
Siobhan Leonard, anti-doping manager with Sports Ireland, wrote to Gearoid Devitt, player welfare administrator at GAA headquarters, informing him of the missing urine samples.
"Four urine samples were collected from three athletes (one athlete provided a second sample due to low specific gravity) and were shipped to a laboratory in Germany.
"Our doping control officer despatched the four urine samples to the laboratory on May 12, 2016, via courier.
"However, the laboratory has informed us that these urine samples did not arrive."
In the second instance, samples were taken from the Westmeath hurling squad on May 7, 2017.
Again, four samples were taken from three players and sent to a laboratory in Germany via the courier.
They were again lost.
Ms Leonard wrote to Mr Devitt informing him: "I am aware that this is the second time urine samples have went missing.
"Sport Ireland does take this matter very seriously and a full investigation has been launched with (the named courier company)."
Sport Ireland said an investigation was launched by the courier company to locate the missing samples, which were not recovered.
"Sport Ireland uses (this company) on average in excess of 1,500 (times) a year and as such occurrences such as this, while very serious, are extremely rare," it said.
Separately, there were also two failed attempts to take samples from Westmeath players.
Westmeath GAA's Aidan Walsh said: "Last minute changes of training venue led to miscommunication with the testers from Sport Ireland."
GAA spokesman Alan Milton said: "On the issue of unsuccessful testing attempts, we have a number of these every year and we try to minimise them."