The HSE says it is "aware" that residents in border counties have been vaccinated in the North, but it's "not possible" to obtain "accurate data".
HSE National Director, Damien McCallion, made the comment in a memo that reveals Co Monaghan still has the lowest uptake of the covid vaccine.
In the memo to Fine Gael TD Colm Burke on foot of a Parliamentary Question, Damien McCallion says that as of the 8th of November, 90.5% of the eligible population, those aged 12 and up, have been fully vaccinated.
He says this is the highest uptake rate in the EU.
Mr McCallion adds that the HSE has "only recently commenced producing vaccination rates by county", and produced county-by-county figures as of October 31st.
It reveals Co Monaghan still has the lowest uptake of the vaccine - 81.2% of the county's eligible population have been fully vaccinated.
However, Mr McCallion says "we are aware in border counties that there will be people resident in the South who have been vaccinated in Northern Ireland".
He added that is it "currently not possible for accurate data on the numbers involved to be provided", despite the HSE "working closely with colleagues in Northern Ireland on this".
Co Donegal has a similar uptake rate, with 81.5% of eligible people there fully vaccinated.
In Co Cavan, 87.3% of those aged 12 and up are fully vaccinated as of October 31st.
Meanwhile, Co Waterford has the highest uptake nationally, at 96.8%.
