I found a "Ruff’s Guide to the Turf" from 1920 in the internet.
According to this book, the Irish Derby, Oaks, St. Leger of 1920 were not run under "classic conditions" (all competitors same weight) but as condition races (penalties and allowances according preceeding success of competitors).
Is this observation true? If yes, when did those Classics get "classic conditions"?
The Irish One Thousand Guineas was a condition race until 1945.
The Irish Two Thousand Guineas has apparently always been a "classic" race.
The Irish Derby was a condition race until 1945.
The Irish Oaks was a condition race until 1914.
The Irish St. Leger was a condition race until 1946.