I run a small bar. Does bar and restaurant inventory management work for spirits I pour by the measure?

Yes. Add each bottle as an ingredient on Menu → Ingredients & suppliers and count it in millilitres, not in bottles, so a 700 ml bottle goes on the shelf as 700. Then on Menu → Recipes & food cost give each drink a recipe, for example 50 ml of gin and 150 ml of tonic. Every drink sold at the POS takes those amounts off the shelf on its own, and the page shows what each drink costs you and what you keep. Do a stock-take once a week by typing what is left in each bottle, and the difference is written down against the count. If the shelf goes negative, the sale still goes through and the number is telling you the pour is bigger than the recipe. The Stock, recipes and suppliers tab in the docs covers units and stock-takes.