The outward journey must be completed on the 'valid from' date
The return journey any day up to and including the 'valid to'
Most Drogheda - Dublin tickets are valid for 5 days, except the discounted day return which isn't available on Friday/Sunday
http://www.railusers.ie/passenger_in... stage1=Submit
If the ticket is not stamped on the outward leg (left side), but is stamped on the return (right side) you clearly cannot use the outward leg
Exit validation will of course catch people reusing the return portions