Also, you can work from a Dublin base for these companies I am fairly sure.
I am just illustrating that there is plenty of opportunities for those who go looking!
Just to illustrate what I meant about sysadmins being a priority for these companies, look at Facebook's job page: https://www.facebook.com/careers/locations/dublin
And Amazon's (note, it's paginated): https://uk-amazon.icims.com/jobs/search?pr=2&searchLocation=...
I wasn't telling people not to investigate these companies, just that their isn't a glut of jobs that their size would suggest. There are companies a fraction of the size hiring more developers than the big guys at any given time.
I'm sure you can sometimes work from a Dublin base for these companies, I'm also sure that would be an exception to the norm.
If you're a sysadmin by trade or want to be (a minority here I think), you could do far worse than apply to Google, Facebook or Amazon.
Unless you're a sysadmin, I don't think there are a whole lot of jobs going in Ireland at Facebook, Twitter or Amazon.