If he held a national ID card, the check-in agent was simply wrong, and he should immediately request compensation from Aer Lingus, as well as from Aviator, the handling agent responsible for Aer Lingus at Arlanda (in other words, the person denying him boarding was an Aviator employee)
Aviator can be contacted at [email protected]
Swedish ID cards are valid across the EU (not to mention across Europe, save for 3 countries).
Are you sure, though, that it was check-in that turned him away, and not Swedish border police?
Because until summer 2015, Swedish law stated that ID cards are only valid in Schengen - as such immigration would not have let you board a direct flight to Ireland, despite Ireland accepting them for entry