I'm slightly hesitant to comment on matters in Ireland as I've never ceased to be surprised just how differently common Directives can be interpreted in practice between there and UK. But here goes!

Reading your post, I get the feeling that you are already well down the risk assessment track and you feel that your case is rather different from that of generalised crane use. The hazards may be similar (falling/swinging load etc.) but your processes are different from the generality of crane operations so you could validly differentiate between them. Similar hazards but different risks.

Why not pick over the syllabus that you have in mind for a formal slingers ticket, suck out the relevant bits and put them into a coherent training programme for your own staff? I would be tempted to ask a slinger trainer to do the editing for you and perhaps oversee the roll-out of your in-house training. (Will this cut across any state-recognised training requirements? If so you've got a political problem not a safety one!) An equivalent example would be FLT driving.

But I'd document it in case of official challenge!