It's perfectly fine under EU legislation for an aircraft commander to authorise a visit to the flight deck while the aircraft is on the ground with engines shut down. Individual company policies differ in minor ways but they largely mirror the law applicable in the country of registration so even in my own company (which being a UK airline tends to be at the most stringent end of the scale on flight deck access), this would be fine. I'm not Ireland based so I don't know the real nitty gritty detail of Irish aviation legislation on flight deck access but it tends to be slightly less prescriptive than the UK.
I think the commander likely wanted to be out of shot out of concern for his personal security / privacy, rather than any concern over company policy. If there was a company policy against access to the FD on the ground, a commander would likely not have let you on the flight deck, and certainly wouldn't have been comfortable with you taking a pic. Just an educated guess but I'm sure you're fine posting it. Unless he told you he didn't want you posting it publicly?