The inflation theory is your answer!
There are many ways to measure distance between Perth and Ireland.
First of all, you should know what kind of distance you want to measure: absolute or relative? this means wither it is in a straight line (which might not be suitable for actually doing it in terms of traveling), or relative, and you would be considering possible routes (boat, plane..).
Secondly, you must choose the point in Ireland.
You can use some online tools that help you with measuring distances. An example is:
https://www.freemaptools.com/how-far-is-it-between-Belfast_Northern-Ireland-and-Perth_Australia.htm
You can then change the cities.
Hope this helps.
