I would see if you can find someone who works in your target companies who can either get you the name of the correct contact in HR or ability to post on internal noticeboards, slack channels etc. You probably might be able to find someone in your contacts who has a contact there I bet - Ireland def feels that small sometimes! Or even just call into reception with a letter or note with the details and see if the reception team will pass it on. I have also seen more and more ads on the local FB pages where I live.