Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
irrĭgātĭo (inr-), ōnis, f. [irrigo], a watering, irrigating: quos (pulvinos) irrigationes abluunt, Varr. R. R. 1, 35, 1: agrorum, Cic. Off. 2, 4, 14: agri, id. de Sen. 15, 53.
With gen. subj.: irrigatione fluminis diluere aliquid, Plin. 36, 12, 17, § 81.
Also transf., refreshment: irrigation ossium tuorum, Vulg. Prov. 3, 8.