There is no "Irish tenor banjo", per se
This post is by Guest Author, Michael Keyes, who sets straight some key issues on our loose usage of the term "Irish tenor banjo" in prior posts. This is a "must-read" if you are considering taking up playing Irish Traditional Music on a tenor banjo.
There is no "Irish tenor banjo", per se. Yes, most Irish traditional banjo players use GDAE tuning (which, by the way is about the same tension as cgda, look up the D'Addario site and see the difference between J63 and J63i string tensions) but the vast majority of players in Ireland use a 19 fret instrument which has a 23" scale. The difference between a jazz tenor banjo tuning and a trad banjo tuning is, as you rightly point out, setup. But there are historical references to the use of GDAE in jazz in the twenties. Early American Irish bands (1935-1950s) used cgda tuned banjos. (See Mick Moloney's site for references.)
A banjo with a 25" scale is a plectrum banjo, an entirely different animal from a tenor banjo. Plectrums are tuned differently, usually like a five string banjo without the fifth string, and are mostly played in chord melody. By the way , there are plenty of Irish musicians using the cgda tuning (Gerry O'Connor being the foremost) played open or capoed up two to daeb which captures the range of a whistle.
The term "Irish tenor banjo" is a marketing tool used by various manufacturers and retailers to sell short scale 17 fret banjos. It has no historical validity. There are, however, plenty of players who enjoy 17 fret banjos in ITM because they are a little easier to play and have a different sound from the 19 fret banjos that they enjoy.