The HSE estimates up to 2,000 cancers have been "lost" as a result of the pandemic.
It's analysed the number of patients diagnosed through its rapid access clinics for breast, lung and prostate cancer.
371 less cases have been diagnosed through these clinics so far this year compared to last year.
Professor Risteárd Ó Laoide, national director of the National Cancer Control Programme, says there are potentially 2000 cases of lost cancer diagnoses compared to last year.
— Michelle Hennessy (@michellehtweet) November 26, 2020
Director of the National Cancer Control Programme Risteárd Ó Laoide said the diagnosed cancer cases are "there somewhere."
The clinic typically finds 21 per cent of cancer cases in Ireland each year.
Mr Ó Laoide is urging people to come forward.
"We want anyone who has suspicion or a worry about cancer to come forward and they will seen," he said.