In the archaeological inventory for the area published in 1986 there is no mention of this piece of rock-art being in a museum. It does however say that is in possession of Oidhreacht Chorca Dhuibhe.
It is described as thus "These 2 stones found in close proximity to one another on the SE slope of Knocknakilton, overlooking the Emlagh river valley. The smaller stone has been removed from the field fence and is now in the possession of Oidhreacht Chorca Dhuibhe.
(1) The measures 1.2m x .8m x .07m to .15m thick. The markings are dominated by a large cup and 3 circles with a radial line running from the central cup through a gap in the inner circle and across the outer circles. A possible cup mark; badly eroded lies between the 2 outer circles. The remainder of the motifs comprise at least 6 cup-marks which, though gapped, may once have been complete as the surface of the stone is spalled where the gap occures"
It appears a cup marked slab still lies at the OS coordinates listed.
Here is a link to some info about the museum in Ballyferriter where it can now be viewed.