Public health chiefs fear that up to 2,000 cancer diagnoses have been missed this year as a result of the Covid-19 pandemic.
The figures to date show that about 90% of cases have been diagnosed this year compared to 2019.
Risteárd Ó Laoide, national director of the National Cancer Control Programme (NCCP), said the issue is "a significant concern", and its impact may not be known for years.
The figure of 2,000 missed cases is based on an analysis of a subset of data for breast, lung, and prostate cancers detected through its rapid access clinics.
The clinic typically finds 21% of cancer cases in Ireland each year.
Professor Ó Laoide said: "The figures to date show that we have diagnosed about 90% of cancers compared to last year.
"This is for [people with] these three cancers — breast, lung, and prostate — who have been referred through these clinics in this manner," he said.
"So in fact, for breast, it's at 100% of last year's figure; for lung, it's at 96%, and for prostate, it's at 65%.
"The figures could be increased as the year goes on. But if we look at our numbers, there are 371 cases... of lost cancers that we haven't diagnosed this year compared to last year, in this subset of patients."
He added: "It's difficult to extrapolate this to the larger number of cancer patients in Ireland.
"Potentially, it could be up to 2,000 cases of missed cancer, which will presumably appear, we hope, later in the year, as we see more patients.
"We know that there are 25,000 cancers a year diagnosed in Ireland in non-melanoma skin cancer diagnosis. So, we do know that there is a gap there."
He added: "On the law of averages, we should have that number of cancer cases, and we haven't had it. So they are there somewhere.
Prof Ó Laoide said that the number of people presenting for cancer screening services dipped during the first lockdown during March and April, but has increased dramatically since.
"What's happened is that we have been catching up, if you like, over the summer," he added.
Prof Ó Laoide said the impact of the missed cases will not be known for a number of years.
He said: "What is the impact of this, we don't know for certain, because we will not know really until we get the cancer registry figures in a couple of years' time — because that will tell us the number of cancers diagnosed, and the stage at which the cancers were diagnosed."
He added: "We won't know really for until a couple of years time can you have some consistency and in-depth figures in the cancer registry."
He said there were a number of caveats related to the figures, because the process of referrals to cancer screening services may have changed during lockdown.
For the low number of prostate cases discovered, at 65%, he said there is typically a delay in these diagnoses, and the figure may increase as the year goes on.
Research conducted by the HSE suggests that a similar impact on services is being seen in Northern Ireland and in Britain.
Prof Ó Laoide said: "We're currently looking at our cancer data and we're comparing it to other jurisdictions, particularly Northern Ireland.
"And we've had Professor Mark Lawlor from Queen's University in Belfast and our initial results suggest that there is a similar impact on cancer services in the north of Ireland and in the United Kingdom as we have had ourselves here."