I think you'd find many Irish registered aircraft are Irish owned. There are, stemming from even before Ireland's days as "Celtic Tiger" hub of international finance, several strong aircraft leasing business based there. Often these companies are reluctant to have their assets registered in client countries because 1) that puts their continuing airworthiness under the scrutiny of a local regulator and 2) it could make repossession difficult in the even the deal turns sour. Kingfisher, anyone?