Before worrying too much about the hydrophilic properties of PLA, it might be worthwhile to test a fitting.
First, print a fitting and see that the freshly made print is strong enough to carry the pressure of the water, and the compression force of hose clamp you may need to connect the stiff irrigation hose to the printed fitting.
Second, soak the printed fitting in water for month or two, perhaps at an elevated temperature to match the higher ground temperature in the summer. You could put the part in a closed mason jar and leave it in the sun. You might add a little salt and fertilizer to the water to simulate ground conditions.
After this aging process, you could test to see if it withstands the pressure of the domestic water system.
You might also measure the ground temperatures where you intend to use the fittings. I find that PLA has no structural strength above about 150 degrees Fahrenheit (65 degrees Centigrade), and the inability to resist slow plastic may start at an even lower temperature. [For example, I print structural PLA parts with negative clearances and then dip them in 160 degree F water to soften them.]
If the printed fittings are strong enough but suffer from water absorption, I would either print them of ABS or coat them with an ABS coating. To make the coating, dissolve ABS in acetone until it is the consistency of thick cream, dip your fitting in the mixture, and allow them to dry. It will take longer than you think it should to dry, and the solution will take more ABS plastic than you might expect.
ABS is not generally considered to be "food safe", but this isn't a potable water system. The FDA lists ABS as conditionally food safe, and I would be comfortable using it to irrigate my lawn and vegetables.