EI-
It is not entirely to do with the fact that the domicile of many lessors is in Ireland, it is due more to problems of enforcing security in certain jurisdictions. In Italy for example, there is a very long a protracted procedure for reposessing aircraft in the event of airline default. The same is true in Russia and some other Eastern European jurisdictions. Some countries do not have mortgage registers, and if the do, enforcement can sometimes be problematic and expensive. Others levy taxes on registration of ownership or security.
The Irish register has become a flag of convenience for many lessors. It permits lessors and financiers to move aircraft relatively quickly if necessary, avoiding the problems of protracted downtimes whilst local courts make up their minds. Local courts often favour the local airline and not the lessor.