Norwegian have DY1 through DY7 Aviation registered as Irish companies; most of the 787s are registered to the latter. They are legally separate from NAI, and Norwegian Air Resources Holdings, and Norwegian Assets, naturally...
So I could imagine the 737s being distributed across those other DY 'companies'. It will be interesting to see the IAA registry update released in a couple of weeks. I should think it would be prudent to keep them separate from the 787 company, just in case long-haul finances don't work out.
It's quite difficult to trace Norwegian's footprint in Ireland as they register their interests through a law firm in Dublin 2, along with several hundred other companies... all sharing the same 'corporate' address.
My interest only arises because I live on the island and it's intriguing to follow the machinations of airlines that register here.