The Collaborative International Dictionary of English v.0.48:

Irrigate \Ir"ri*gate\, v. t. [imp. & p. p. Irrigated; p. pr. &
   vb. n. Irrigating.] [L. irrigatus, p. p. of irrigare to
   irrigate: ir- in + rigare to water; prob. akin to E. rain.
   See Rain.]
   [1913 Webster]
   1. To water; to wet; to moisten with running or dropping
      water; to bedew.
      [1913 Webster]

   2. (Agric.) To water, as land, by causing a stream to flow
      upon, over, or through it, as in artificial channels.
      [1913 Webster]

   3. (Med.) To rinse (a wound, infected area, etc.) with a flow
      or spray of a liquid.
      [PJC]

WordNet (r) 3.0 (2006):

irrigate
    v 1: supply with water, as with channels or ditches or streams;
         "Water the fields" [syn: water, irrigate]
    2: supply with a constant flow or sprinkling of some liquid, for
       the purpose of cooling, cleansing, or disinfecting; "irrigate
       the wound"

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "irrigate":
   adulterate, asperge, attenuate, baptize, bath, bathe, bedew,
   bespatter, besprinkle, cut, dabble, damp, dampen, dash, dew,
   dilute, douche, flush, flush out, gargle, holystone, hose,
   hose down, humect, humectate, humidify, lather, launder, lave,
   moisten, mop, mop up, paddle, rarefy, reduce, rinse, rinse out,
   ritually immerse, scour, scrub, scrub up, shampoo, shower, slobber,
   slop, slosh, sluice, sluice out, soap, sparge, spatter, splash,
   splatter, sponge, spray, sprinkle, swab, swash, syringe, thin,
   toivel, tub, wash, wash out, wash up, water, water down, wet,
   wet down