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.
Some 371 less cases have been diagnosed through these clinics so far this year compared to last.
The clinic typically finds 21% of cancer cases in Ireland each year.
Director of the National Cancer Control Programme, Risteard O'Laoide, is urging people with symptoms to contact their GP.
"On the law of averages we should have that number of cancer cases, and we haven't had it.
"So they're somewhere.
"We want anyone who has any suspicion or worry about cancer to come forward and they will be seen".