There is validity to both sides of the argument. Greenways on a former railway bed that will never be used again for rail transport make a lot of sense. However if the land was sold to farmers they could have valid concerns about security, trespass and liability, which may or may not be addressable. Many urban folk simply don't comprehend the potential issues which is understandable. A train passing through a farm poses little risk as the humans are contained, however folk walking and cycling may pose risks unless restricted to the greenway route.
In principle I am in favour of greenways for Ireland inc, as they can be a wonderful local and tourist amenity. Providing however that they are not being built on former lines that could be reopened in the future as viable extensions to the transport system (e.g. parts of the western rail corridor). Some closed railway lines are still owned by the state and others not.
The 'hiker' recently awarded €40k compensation in the high court has put the "cat amongst the pigeons" as regards liability arising out of public land access, and also famers who have facilitated access on some non right-of-way routes across private farm lands. All solvable with a good dose of common sense, but that is rarely applied when the "fear" card is unleashed.