From the middle of the 18th century a glass manufacturing industry flourished in Ireland , mainly because the excise act did not apply there.  Thick glass was made and, as the style of cutting and engraving was popular, the finished glasses could be cut deeply with attractive patterns.

In the early 19th century new styles of drinking glass appeared, and today these can be found quite cheaply, though undamaged  sets are more expensive.
There are a number of ways of telling fakes from originals, but it really needs more than the application of one or more of the rules below, and in the long run the advice of an expert will be needed.

Up to about 1750, when glass was blown, a jagged surface was left on the base where the pipe was broken off, this is called the pontil mark.  After 1750, glass makers tended to grind the edges away to finish the glass properly, and in the 19th century little or no trace of the pontil mark could be found on any glasses.  These pontils can be faked, and faked well, look for scratches on the base, which suggest years of being drawn across surfaces.  But these scratches can also be faked by using emery paper, but the scratch lines will be straight or circular following the action of the abrasive used, where the scratches of time and usage should be uneven.

The opaque twist of the 18th century can be copied, the enamel used in the real article was dense white, with a touch of blue at the end of the twist.  Copies made in the 19th century have a wishy washy look, and the spacing between the spirals are uneven.  One way to get judgment in these matters is to examine known fakes beside real known pieces, the difference will become know quickly, but if you cant trust your judgment, the glass can be examined by special light rays or ultra violet light.  Glass in the 18th century has a stress pattern that is different to those of 19th and 20th century glass.

You don't have to collect only glasses , you could add to this by collecting decanters or bottles as these have been made in great quantities over the years.  The earliest bottles where made at the turn of the 17th century, and upon 1720 they where generally made in one shape, a globe with a flat bottom, short neck and untidy lip.  Very little care was given to proportions.  The colouring was a type of green or brown.

After 1720 the globes thinned out to become cylindrical and the necks made longer,  along with the bottle came the decanter,  at first the decanter was globular in shape and had a longish neck, the stopper had a ground edge to fit into the inside of the neck, which was also ground.  Early stoppers where spire shaped.  By the middle of the 18th century the globes had become cylindrical the same shape as the bottle.

Round about now people started to add engraving and ornament the decanters with enamel, add the name of the drink for which it was used such as port or claret.  Or add slogans.  One of the best know decanters of this time would be William Beilby, of Newcastle upon Tyne and pieces decorated by him are in great demand.  The contents of the decanter was later indicated by the use of silver labels engraved and hung round the neck, these in them selves are now highly collectable.

Two other decanters where made later, both which are still relatively easy to find, these were the barrel shape and the square shape which generally have a ball top stopper of cut glass in diamond shaped faucets.  The square decanters where copied a great deal in the 19th century, and appeared in sets of three in a stand with a bar which prevented butlers taking them from the stand when the lord of the house was not there, these were called tantili.

Irish made decanters are especially interesting to collectors, for they where finely made. Four main factories produced a lot of good vessels,  The Cork Glass Co, The Waterloo Co, and those at Waterford and Belfast.  These decanters where usually of a barrel shape, wider at the top than at the base, with three rings round the neck and a mushroom stopper, cut in flutes. 

Although the basic design was similar each factory had it own individual engraved decoration.  The high mark of Irish glass, was short lived from about 1770 to 1825 for in the latter year of the Glass Excise act of England was extended to include Ireland.