I created a script for XChat according to this answer, and I was wondering if there is anyway to apply XChat icon to it.
Create a .desktop file like this and put it in /usr/share/applications/
[Desktop Entry] Type=Application Name=XChat Firefox Exec=/usr/bin/your_script Icon=/usr/share/icons/xchats_icon
"XChat Firefox" will then be a launchable application with XChat's icon.
Exec can also take a bash command (with arguments, optionally) as its value.
For a complete explanation of the .desktop format, see this specification.