Yes thought it is probably a married segments. Their booking engine seems fairly solid at showing what can actually be ticketed, so maybe their system can't do it. It might work with FRA as a stopover though, I'll take a look.You can try with the call centre, but it looks like it’s married segments. About the best I can find is DUB-IST-SIN-SYD, TK/SQ/SQ. But awful transit times in IST (13+ hours, with a visa required if on an aussie passport and you want to leave the airport).
You can also try ADL and BNE, again with the TK connection, the call centre should then be able to add a VA sector to SYD. Depends on the dates you’re looking for. ADL has very short connection times in IST, like just under two hours, but SIN-ADL can be hard to find in business.
Eurostar to Brussels is only 2hr30. Do the Ireland leg in the middle then fly home from a European port instead to keep the $1000 the UK tries to theive off pax