the prefix for irrigate is

The verb of irrigation is irrigate.

As in "to irrigate something".

I had to irrigate the fields today.

Irrigate is already a verb because it is an action.
Other verbs are irrigates, irrigating and irrigated, depending on tense.

For example, here are some sentences:
"We will irrigate the crops today".
"They irrigate the fields".
"We need to do some irrigating".
"They irrigated the farmland"

The Japanese learned to irrigate from the Dutch people of the Dutch enclave of Dejima. Japan learned how to irrigate during 1641.