[Proj] How to use +towgs84

[Aaron Friesen]

Everyone,

I'm trying to familiarize myself with proj.

I'm trying to use cs2cs to do a datum shift where a 7-parm
transformation is used.  I want it to do the shift in
both directions.

"proj -ld" seems to provide a possible example.  The listing
for ire65 has example towgs84 settings.

Datum:      ire65
Ellipse:    modif_airy (sic)
Comment:	towgs84=482.530,-130.596,564.557,-1.042,-0.214,-0.631,8.15

What do I need to specify for cs2cs for both directions
of the transform?

+proj=latlong +ellps=mod_airy

+to

+proj=latlong +ellps=WGS84

seems like a resonable start.  Where does the +towgs84 argument go?
How do things change in the reverse direction?

Thanks for your assistance,

Aaron