There are several reasons for Irish registry. Originally the registry was simplified when Guinness Peat had its leasing business there. Then during the 80's-90's Ireland wanted to attract more business that would domicile there while operating/owned in Europe, or those offshore companies that wanted a European presence, all part of the Celtic Tiger (let's not talk about how the tiger drowned...) growth strategy.
Ireland has good double taxation treaties within Europe, secures the ownership of the leasing company, IFLC uses it a lot too for that reason. Furthermore Ireland allows foreign, think offshore companies, to register anything from Aircraft to financial funds.
So don't think that Norwegian would just use the Irish register to circumvent local employment laws, it is purely done as it is the most favourable and affordable registry in Europe.