It will be installed automatically if you install PHP5.x with Plesk autoinstaller:
[root@ppu12-5 ~]# rpm -qa | grep ioncube
php5-ioncube-loader-5.0.12-centos6.15072015.i386
plesk-php52-ioncube-loader-5.0.18-centos6.15101218.i686
plesk-php55-ioncube-loader-5.0.18-centos6.15101215.i686
plesk-php53-ioncube-loader-5.0.18-centos6.15101218.i686
plesk-php54-ioncube-loader-5.0.18-centos6.15101215.i686
plesk-php56-ioncube-loader-5.0.18-centos6.15101214.i686