There has been no freight on the Sligo line since 2008, so don't hold your breath! And chances are, if there's been ballast trains operating over the line in recent months, their work is probably done now and so are unlikely to make further appearances for a while.
If you're based in Mullingar and have access to a car (or if you take Bus Éireann route 70), pop down to Athlone, where you'll get to see the liners and timbers passing through. You'll also be able to see ICR services splitting there, with the separated portions then heading onto the Galway and Westport lines.