What would you do in my situation, brainfart?
I don't know.
Make a big list with all the facts. Investigate the tax situation, is there a double taxation agreement between Ireland and Australia? I assume that if you invest in Ireland you will have to pay some taxes there, what other taxes will be due in Australia?
Maybe invest about equal sums in both countries for a year or two in the same or substantially similar funds/ETFs/whatever, decide which country is better and then move all your money. I don't think that you will take a big financial hit after such a short period. Most likely Australia will win, and even if it doesn't and is a close second it might be less complicated and therefore better.
Another option: 30K isn't all that much compared to the money you hope to have in 20 years, so invest those 30K in one (MSCI ACWI or similar broad index) or two (e.g. 70% MSCI World + 30% EM or similar) cheap, reinvesting, possibly synthetic and tax efficient ETFs and simply let them grow unmolested. Don't touch them unless you absolutely have to, except for yearly rebalancing, maybe keep some cash in there for travel and other expenses and let time do its magic. With the right ETFs and bank/broker you will be due zero taxes and fees as long as you leave them alone. Worry about withdrawal strategies later when the need arises. You don't know what the future brings and how taxation will change anyway so don't worry about that now.
Even if this will produce less than optimal results you added some kind of diversification and will have more options in the future if you ever decide to return to Europe.
Then concentrate your efforts on your Australian investments where you can do all the complicated strategies involving 7 different funds and strategic overweighting of sectors etc ;)