Basil,
Fine, just flag him up for an interview on arrival.
Not helpful - It's an issue of sensitivity.
I would have reacted just as this guy did. Arriving back to one's own sovereign soil, one doesn't expect to be questioned by officials from another state. I imagine you might see red if a French official attempted to interview you after arriving back to the UK from somewhere.
As I understand it, the guy was let through when he questioned under what section of Irish law they were operating (which appeared to be a bit adhoc).