Origin 1650, from Latin irriguus, equivalent to irrigare "to wet" + -uus
Example from 1667: Paradise Lost by John Milton:
A happy rural seat of various view:
Groves whose rich trees wept odorous gums and balm;
Others whose fruit, burnished with golden rind,
Hung amiable - Hesperian fables true.
If true, here only - and of delicious taste.
Betwixt them lawns, or level downs, and flocks
Gracing the tender herb, were interposed,
Or palmy hillock; or the flowery lap
Of some irriguous valley spread her store,
Flowers of all hue, and without thorn the rose.
Another side, umbrageous grots and caves
Of cool recess, o'er which the mantling vine
Lays forth her purple grape, and gently creeps
Luxuriant: meanwhile murmuring waters fall
Down the slope hills, dispersed, or in a lake,
That to the fringed bank with myrtle crowned
Her crystal mirror holds, unite their streams.