I think the relevant modification would be in the button travel: the better two-row boxes set up for Irish playing have buttons that are only minimally proud of the board, and downward travel is limited as well.
I don't know if the older melodeon tradition was based around the full-throated, wide-open clapper of the Cajun accordion. Another question would be if the reeds are LMMH like a Cajun box, or LMMM like you see on some Paolo Sopranis.
Randy Falcon built a >less air = more speed< box for Ray Abshire, but you can't get any more expensive than that! (except maybe a Messevier from Quebec)
My Bon Cajun sounds beautiful when I use just the two mid-range reeds to play the exactly three Irish tunes I know.
I'd ask some of the players who are doing single-row Irish what their reed setup is.
There are things people don't uderstand about cajun boxes. 99% are 4 reed boxes excepting Falcon boxes which are 3 reed boxes for the dual key ones. Some Irish boxes are only 2 reed boxes. Just remember you can close the stops off on the cajun boxes and and make them two reed boxes. Irish boxes made in Quebec haved only 2 or 3 sets of middle reeds most of the time and are not necessarily faster than cajun boxes. Button travel can be adjusted on both. Usually you will find that boxes with less reeds have a somewhat faster responce.
Irish melodeon players use a variety of one rows from many sources.
The Great Johnny Connolly used Hohners for years until he got a hand made box. His music is no different.
Brendan Begley uses a Castagnari melodeon (MAX)
These are excellent boxes but a clumsy air button arrangement.
Other players are using Quebec made boxes and some are using USA made boxes retuned and the Castagnaris.
I have owned all of the above and I play primarily Quebec and Irish music.
It is a rare Irish player that has much of a tremolo
most are dry tuned or a whisper of tremolo.
Equal tempered..yes.
However in traditional Quebec playing (until recent years..) one middle reed was 5 cents flat and one reed was 5 cents sharp..so playing Irish sounded odd.
If you get a Quebec box specifiy A 440
I got a handmade box tuned that way and though rare and expensive , never liked it. It was retuned and was a brilliant box. Of course Binci reeds and a handmade bellows helped along with some other details peculiar to Quebec boxes and the particular maker.
My recommendation is to go for the top end and check out Melodie in Quebec.. or retune a Hohner..
Yes a lowly 114... lots of great music has been made on these.
Just as the Quebec guys specialize in Quebec boxes the Cajun builders specialize in Cajun boxes
And Quebec boxes are closer to what the Irish players are looking for without the retuning.
I do not recommend the Castagnari other than it is well made and very light and very fast and very small... it's just that air button and the Euro exchange.
As to lower button height, flapper placement and so on .. incidental but may be true.
I Totally agree with Jefe excepting that if you order a Louisiana made box and specify Irish tuning no retuning will be necessary. However, Irish boxes I have seen did not have the bassoon or piccolo reeds. Just two or three sets of middle reeds. Please note however that I have not seen everything.