The HSE says "a constrained" environment has contributed to its problems with waiting lists for tests.
Over the weekend, it emerged that 27,000 are waiting on MRI scans, with those numbers not appearing on official statistics.
The HSE has denied hiding the figures, and said it is undertaking a "full review" of waiting times for diagnostics.
Some 2,000 people are waiting over two years for a scan.
The Assistant National Director of the HSE's Acute Hospitals Division, Angela Fitzgerald, said there are a number of reasons.
"The first thing is that we've been in a constrained environment - our ability to expand the manpower to run the capacity out of hours has been constrained."
"The expansion on the working day from 8[am] to 8[pm] has allowed us to extend our practice, but we also need doctors available to do that."