The best way to tap into a PVC mainline (from a long-term reliability stand point) is to just glue in a standard PVC Tee fitting.
If you can uncover enough pipe, you cut out a section something on the order of 1-1/2" long, glue the Tee to one side, then bend the pipes just enough to glue the other side in.
If you cant uncover enough pipe for bending, you can also use the trick of 4 90 degree elbows. For this trick, you cut out a section of pipe on the order of 1 foot. You then glue 90 degree elbows to the two exposed pipe ends, pointing them both to the left or right (they must line up exactly). You then connect two 90 degree elbows to a Tee. The tough part is getting these elbows to be exactly the same distance apart as the two elbows on the mainline. But once you've done that, you then glue the two pairs of 90 degree elbows together. The result would look something like this...
-----+*****+---- <-- Existing Mainline
.....|.....|.... (Dots to force spacing of image)
.....+--+--+.... (* denotes cut-out mainline)
........|....... <-- Irrigation Ty-in