the short answer is that in the absence of a bottle being marked Penrose, Waterloo, Cork etc., it's impossible to be certain - these are attractive - a good purchase. I think that if it was easy then we'd all know what to go out and look for.
The only personal comment that we might say with these, is that the midriff band of relief diamonds is unusual for an Irish bottle from the period we're discussing - not that I'm remotely expert, but having flicked through Phelps Warren decanters with this feature appear very uncommon. Plenty of finger bottoms, mushroom stoppers and triple band neck rings - it's likely only the vesica motif is a reliable decorative feature which points, but doesn't guarantee, an Irish provenance.
Take it that the pontil depression is shallow and very wide?