I've witnessed the RPU try the heavy treatment routine, demand details of a third party etc.
While authorised officers (i.e. ticket inspectors, management and some others) have the power to detain under the transport act, this applies only to a very limited range of situations relating to the railway, trespass, fare evasion etc.
Staff are under very clear orders that while they legally are empowered to detain fare evaders they have been told by management not to. Staff have correctly detained persons on several occasions, two incidents in Bray come to mind, the guy who went on a window smashing binge was detained by a driver and station staff, also at Bray again a driver and station staff tackled 3 fare evaders who tried to make a run for it (driver ended up in hospital afterwards). In both cases the gardai arrested the individuals and there was a successful prosecution.