Question
I want to cover a bank with rug junipers and with drip lines for irrigation.  Question, how far apart should I run the lines vertically, and how far apart horizontally should I plant the junipers on each line ??

Answer
These folks suggest 4 ft. to 6 ft. apart:

htp://landscaping.about.com/od/groundcovervines1/p/blue_juniper.htm

In our hot, great basin climate, I bunch them up closer to 3 feet apart.  Irrigation-wise, don't  let the drip line individual zones or "runs" get longer than 300 feet, or you'll have pressure problems.  

With drip, you run the tube to the individual plants, so spacing "vertically" depends on your spacing pattern.   You'll also want to have the lines run horizontally (cross-slope), for better water distribution.  Running lines vertically (down the face of the slope) creates more pressure at the toe of the slope than at the top. Otherwise, you'll over-water the toe and under-water the top of the slope, or worse; you may blow out the lines at the bottom.

Your irrigation point of connection should be at the top of the slope, or you MUST install an appropriate backflow preventer.  Check your local codes - Ours requires a double-detector check valve or DDC.

I'd stagger the plantings so that they form "triangles" between the rows, by offseting the spacing, like so:

*      *      *      *      *      *      *
  *      *      *      *      *      *
*      *      *      *      *      *      *

Other good recommendations that relate to your situation:
http://www.dripirrigation.ca/HowTo_Plan3.asp

Hope this helps you!  Good luck ~M