Their only legal address is in the US. I'm in the UK, and stopped short of looking for a solicitor who would be able to handle this. I'm still considering filing a complaint with the UK Telecoms regulator 'Ofcom'.
The punchline is that the American collections agency they use actually do respond to email and (I believe) dealt with it. But for it to go as far as a collections agency was nuts.
That shouldn't stop you from contacting Google Ireland. They are part of the same company and are their local legal presence after all. If you send them a certified letter and they sign for it, that's it.
What about sending them a physical, certified letter? Have your lawyer write it.