Ant で Ivy の jar をダウンロードさせて、その jar からタスクを定義する ref: http://qiita.com/items/21e5370ffa178bb19bc9

build.xml

<project xmlns:ivy="antlib:org.apache.ivy.ant">
    <condition property="ivy.home" value="${env.IVY_HOME}">
        <isset property="env.IVY_HOME" />
    </condition>
    <property name="ivy.home" value="${user.home}/.ivy2" />
    <property name="ivy.install.dir" value="${ivy.home}/lib" />
    <property name="ivy.install.dest" value="${ivy.install.dir}/ivy.jar" />

    <target name="-check-ivy-user-installed" description="check ivy jar file existence on user's ivy home">
        <available file="${ivy.install.dest}" property="ivy.install.dest.exisits" />
    </target>

    <target name="-taskdef-user-ivy" depends="-check-ivy-user-installed" if="ivy.install.dest.exisits">
        <path id="ivy.install.path">
            <fileset dir="${ivy.install.dir}" includes="*.jar" />
        </path>
        <taskdef resource="org/apache/ivy/ant/antlib.xml"
                 uri="antlib:org.apache.ivy.ant" classpathref="ivy.install.path"/>
    </target>

    <target name="install-ivy" description="Download Ivy jar file to ivy home">
        <property name="ivy.install.version" value="2.2.0" />
        <property name="ivy.install.src"
            value="http://repo1.maven.org/maven2/org/apache/ivy/ivy/${ivy.install.version}/ivy-${ivy.install.version}.jar" />

        <mkdir dir="${ivy.install.dir}" />
        <get src="${ivy.install.src}" dest="${ivy.install.dest}" usetimestamp="true"/>
    </target>

    <target name="clean-ivy" description="Delete Ivy jar file from ivy home">
        <delete file="${ivy.install.dest}" />
    </target>

    <!-- sample task to use user ivy -->
    <target name="resolve" description="Resolve dependency" depends="-taskdef-user-ivy">
        <ivy:resolve />
    </target>
</project>

usage

ant install-ivy