foo slash bar.clj

(ns foo.bar)

(defn some-fn [x]
  (println x))

Main.java

import clojure.lang.RT;
import clojure.lang.Symbol;

public class Main {

    public static void main(String[] args) {
        // (require 'foo.bar)
        // (foo.bar/some-fn "Hello interop!")
        RT.var("clojure.core", "require").invoke(Symbol.intern("foo.bar"));
        RT.var("foo.bar", "some-fn").invoke("Hello interop!"); 
    }

}