Given that the sign in question is in Southwest Scotland and not Ireland I'm not sure that Irish land laws apply :-p
There is a similar much earlier act that sets forth landowner liability in Scotland but the Land Reform Act Scotland 2003 introduced the right to roam and puts some limitations on it, so the "unauthorised access is prohibited" bit is bunkum. Where liability applies: if a roamer trod on e.g. a trap then the landowner would be subject to criminal charges, if someone fell into a ravine because of a missing fence or signpost then it would probably be dealt with on a case by case basis.
Ha, not that kind of bog. Crossed a field and only discovered how close the groundwater was to the surface when I was halfway across. The cows in the field started converging on my position and even though they might have been placid, when 20 tons of beef is watching your every move and cantering towards you while you're up to your knees in mud it is better to flee than to see what they want
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