If we have no deal

The Irish gardaí are going to literally spend their whole time arresting people for driving on a UK passport

So it seems like a good time to get into organised crime and that

Almost right.  The Gardaí will be arresting everyone who is driving on a UK passport AND who is NOT a cousin of theirs or a prominent local farmer, a publican, another Guard or a TD.  

So only English tourists really.  

OK maybe it's different up in Dublin, but where I am the Guards are not exactly given to doing it by the book if they know the person they are dealing with.  

Isn't the international driving thing based on something that's not EU membership? So would remain post-no deal brexit? I can drive on either of my two licences in most countries as a visitor. I thought there was some sort of convention thingy.

Also, isn't it your driving licence that gives you permission to drive in another country, not your passport? 

And yes, I hold two licences. And yes, it's probably unlawful in both places. Don't tell the SRA.

That's just weird. The German equivalent of the Old Bill were happy enough with my UK licence when they stopped me for a chat in the Black Forest a few years back...

You get 1 year's grace, Bernstein (assuming you're resident). At least, in the states that I'm aware of. After that, you're supposed to hand your UK one in and get an Australian state one. They are supposed to send it back to the DVLA, but my personal experience is that that system has failed.