Just bought 2004 Prius Dublin Ireland

I can't quite completely understand your post. You know what they say about the UK and US being divided by a common language. Another period or two might help, too.

If you're getting a red triangle, the check engine light should be lit as well. That means there are diagnostic test codes present and a Toyota scan tool may be needed to read those. Without those codes we are just guessing.

And there are two batteries--presumably you changed the 12V accessory battery, not the large hybrid traction battery? At twelve years old, and after a long period of not being used, it's possible that there's a fault in the hybrid battery.

Also be extremely careful when jump starting the Prius. A careless mistake could essentially ruin the car.

 

congrats and welcome, all the best!

 

Do as post #2 says!

There are codes waiting to be read, which will begin to point you in the direction of a possible repair.

Best to get miniVCI w/ hacked Techstream from Amazon/eBay.

 

Based upon your post, I understand that the Prius will become READY after the second time you try to start the car. Then what happens?

Can the car be driven?
If yes, what fuel economy does the car register and does it display any driveability issues?

Are any warning lights or messages still on? If yes, take sharp photos and post. If warning lights remain on, then you need to retrieve the DTC. Use Mini VCI for that purpose.