I was about to suggest a response consisting of a shotgun to the backsides of these scumbags, but I see you specifically said "What action can we
legally take". Ah well.
Anyway, the scumbags probably can't read so the private property notices will not make any difference; chances are they are going to ignore those anyway.
Sounds like they've performed criminal damage on the fence. However you can't prove who actually did it. They can be caught for traspassing if the gardai are quick enough to get there, but chances are the gardai will do nothing with them and you'll just bring aggrevation on yourselves - they may start targeting cars in the block as "revenge".
And "detain them"? What planet are you on! Yeah try that and let me know what flowers you want sent to your hospital bed!
People have a duty of care to themselves. Can't imagine a judge blaming the owner if kids scramble up onto the roof without permission. However putting up a notice saying "danger" or something would help their case I would think (make sure to get pictures of the setup - roof showing danger sign etc.)
My suggestions are:
1) get the fence repaired and boost it up with barbed wire etc.; same with the sheds; Is it possible to plant thorny hedgerow?
2) get a private security company to patrol
3) See if the council can make an alternative path to whereever they are taking a shortcut to
Alternatively unless it's causing real problems, you may just have to grin and bear it...