Joseph O'Kelly’s waltz displays a sophisticated harmonic language typical of the Irish-French romantic school. It stands out for its graceful ornamental figures and elegant melodic contours.
Precision in the execution of the ornamental turns is essential for the character of the piece.
Comparable difficulty and character — good candidates for what to learn next.
Au bal (Op.51) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Au bal (Op.51) lasts about 5 minutes.
Au bal (Op.51) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Berceuse by Joseph O'Kelly, 'S Wonderful by George Gershwin, Bella Bocca by Emile Waldteufel — see the full similar-pieces list on this page.