What is this Irish idiom?

The idea of filling your boots' almost certainly dates back to the days when a victorious army would plunder the places they had recently attacked and defeated. Obviously, the soldiers would stuff their pockets, saddle-bags or whatever and then - so as not to miss anything - even their boots with booty...if you'll forgive the choice of word! Soldiers - particularly cavalrymen - in past times often wore wide thigh-length boots which were quite roomy! That's why we have the modern sense of "take full advantage" when we use the saying.

You know, here's my best guess.

"Fill your boots" just might mean "Do it!" or "Get on with it!"

You have to put your shoes on to go out the door and get something done.

And if you are going to fill someone else's shoes, you have to be able to do the work that person does.

Without any more context, I can't tell you any more.

It means to grasp the opportunity while you can.

Informally to take over someone's role and fulfil it satisfactorily.

Some bogger saying, we don't use phrases like that in Dublin