HDPE Long Term Hydrostatic Strength

Irrigation pipe is only pressurized when it is delivering water.

Not sure what you need here. The pipe is not normally subjected to high pressure in irrigation applications, in any case. 50PSI around here.

The drip system at my current office has been is service for 15 years and it's still functional.

Ryan-Herco, Harrington and others have this data on their web sites.

Do you have a special application in mind?

It depends on the diameter and the wall thickness, neither of which was given.

Also, the Maximum Working Pressure rating also is highly dependent on the actual ASTM material spec and grade. This can vary dramatically from one HDPE pipe material to the next. Check with the actual manufacturer of the pipe or tube to obtain this data. BTW, most ASTM numbers and grades are stamped along the barrel of the pipe or tube....and most times the Max. WP and Max. working temperature are right there next to the ASTM data.

All you have to do is look!

You have not noted exactly what sort of "irrigation" service you are dealing with; however, working on "Temperature" that the Capt. mentioned you should probably (particularly considering your location) be aware that the strength of hdpe is substantially de-rated at above room temperature. Per e.g. http://www.ips-frp.com/B5TechBulletins/TechBulletins/HDPE%20Versus%20FRP.pdf :

"It should be noted that surface temperatures in HDPE pipe exposed to direct sunlight exceed 150ºF. HDPE is colored black to offer maximum resistance to stress cracking which is a common failure mode for this product. The black color increases pipe temperature from radiant heat absorption.

HDPE pipe is rated at 70ºF. HDPE mechanical properties fall off immediately above 70ºF with about 1/2 the strength at 150ºF."