Most certainly is, StevieB. Two RPSI locos have worked there, and at the time of the Antrim relaying contract some years ago, the DCDR had offered to make its two E class locos available should it be necessary (it wasn't).
A J15, though, is too big for the DCDR. Not in terms of size, of course, as the DCDR was a main line, and the Quoile Bridge could take an 071 - but coal appetite. It would cost several thousand each way to transport it to Downpatrick, and its coal consumption would be far too uneconomic.
In theory, a straight swop could be done with 142 and 146!