Have to say I remain sceptic re the alleged provenance and age which have been attributed to this bottle, though I admit to being anything other than clever when it comes to Anglo-Irish bottles from that late Georgian period. I'm well aware of the almost identical appearance of this one to the quoted similar decanter in McConnell, to which he assigns a date of c. 1805 - although there is a small difference of slanting blazes on the book example - this is assuming I'm looking at the correct image. In fact I don't have the later Decanter book which believe Laird is possibly referring to - mine is the earlier McConnell volume (2004), but think we're speaking of the same bottle. IIRC, McConnell appears not to provide a specific glass house for the bottle in question, simply commenting " Anglo-Irish Prussian Decanter".
We are back to the proverbial problem of making an attribution etc. on something we can only see on the screen - this may well be a genuine period example - though IMHO dating based on appearance of cutting style alone is very unreliable. The cutting style is what we assume tells us most about a date, and in truth it should, but with decanters it's often the least helpful since there are more copies around than you can shake a stick at - mostly produced between c. 1870 and 1930 (what McConnell calls 'endemic plagiarism'), and yes, this particular style of triple neck ring has been copied by other glass houses.
Phelps Warren discusses much of the early history of The Waterford Glass House - the Penrose family members started the business in 1783 selling out to 'Gatchell and his partners' in 1799 - and it's for this reason that the early years of the Company's history are referred to as Penrose/Waterford - the Company ceased in 1851, re-appearing in the mid C20. Most of the known Penrose period decanters look to have engraved decoration rather than cutting, and if you didn't know, then looking cold at this bottle and the McConnell example, neither has what might be thought of as typical late C18 Irish engraved decoration, and the legendary 'blue' tint aside, I would have thought that some degree of typical lead colouring to the metal would have been expected.
I don't think this decanter is a dip-mould made piece - perhaps that sort all have a kick in the base - but I'd have thought there might have been a radial cut star on the base with a central ground depression. Would also agree that matching Nos. are a feature from later in the C19, though the replacement stopper is of no consequence here. I think it's only the early dip-mould pieces that carry the glass house name on the base.
A bottle that is assumed to be over 200 years old will have definite signs of age related wear, both inside and out - and still waiting to see a photo of the underneath please.

P.S. Phelps Warren is a stunning good read if you're at all into C18 and C19 Irish glass - the later revised edition from 1981 (not the 1970 job)