Farm Legalities

Of course they can sue, they need not even be climbing, they could rest against the gate or just hold the top bar to get a gash.

20k min in compo, maybe a bit less if it doesn't go through Insurance.

your only option is a sign indicating private property no trespass, then if you catch them within the property call the Guards and have them prosecuted for trespass

There's very little you can do, even the barbed wire on the gate isn't advisable. you should have a good public liability insurance on the field and then if they want to sue you don't have to worry as the insurance company will look after it

https://www.boards.ie/discussion/2058204070/field-security