Hi Marion Louise,
I can help you with this one having done the Jordan trip in April of this year.
You don't need to take USD to change into Jordanian Dinar as you can change GBP easily, and at a better rate than you will get in UK.
In Amman there is a small general store about 5 minutes walk from the Bristol Hotel. Turn right out of the hotel and walk up to the first junction. Turn right again and a few yards along on the right-hand side is the general store where they will change GBP for you. The rate at the time was 1.05 compared to 0.98 offered in UK. You can also take the opportunity to buy your bottled water and any other provisions you may require from there
.
There is also a good exchange facility in Petra at the Petra visitor centre, where the rate was slightly higher than that in Amman. To find the exchange you need to walk to the far left-hand corner of the square that comprises the visitor centre. The exchange is then on the right-hand side by a flight of steps. You have to walk this way to get to the Petra site itself so you can't miss it.
Bob, I know that you are from Ireland so I would say that you shouldn't have any problems changing Euros either. At the Petra exchange they had a big list of currencies on the wall showing individual rates for that particular day and I'm fairly sure that the Euro was on there.
So to summarise, if you want to take a small amount of Dinar from UK (or Ireland) to get you started then that's fine but not essential, and you can take your local currency to change while you're there. You certainly don't need to change GBP/Euro to USD and then change again to Jordanian Dinar.
Hope that this answers your question and if you need to know anything else just ask.
Have a great holiday
Mike