Hello there
Eoin may answer for himself, but the reason you want a short walk to Glendalough is because the monastic city is such an amazing place that it would be a shame not to have plenty of time to explore it. Some people spend two nights there, just to have a full day for it.
For Iron Bridge-Stranakelly, the terrain is easy (fairly flat and lots of road walking) so it isn't hard to make swift progress, whereas in the northern sections, you are crossing mountainous country with some boggy bits.
Finally, a comment: some people regard the southern portion of the Way as so dominated by road walking that they don't bother.

There is no shame in terminating your walk at Tinahely if you feel this way. (I walked every inch, but I was researching it for the guidebook. If I had been walking for pleasure only, I think I'd have stopped at Tinahely which is a lovely village and would reduce your final day from Iron Bridge from 31 km to 21 km. This might make the difference to making it easy to get back to Dublin the same day. Or, if the weather turns wet, it may be useful information to have up your sleeve.
