About three weeks ago, I purchased a Hohner Cassotto 26 and a Hohner Cassotto 27 from Ebay to round out my collection. I had to tweak the water valve on one, but both were in pristine condition. I paid about $15 for the soprano and about $60 for the alto. The owner of the alto gave me a $40 dollars off when I explained that water valve leaked. So I got two great melodicas for about $35. My wife is going to kill me if I buy one more.
The sound is very smooth, no high harmonics, and very hollow. It is difficult to describe, but I like it. The closest thing I can think of to compare it to would be the timbre of a flute playing in the lowest register. The sound will not fit every song, but as Melodica-Me suggests, it will probably be perfect for some songs where a regular sounding melodica will not fit.
I am not an expert in Irish traditional music, but what little I have heard of this traditional music would suggest to me this is not the sound you want for it.
I sense these cassottos where often used for education. Both the cassottos I purchased had names written on the cases and on the back of the instruments. Both were hardly used.
By the way, the volume of these cassottos seem a bit softer than most melodicas, but that could be because the high harmonics are not there. It seems like a weaker sound; not a sound that is going to play the role of horns or a harmonica. I think the cassotto would be great for ballads and softer gentler music. Maybe some Americana music. Could be good for some jazz pieces and formal numbers. Similar to the Hohner Piano 26/27/32, they require a bit more air to make them run.
Melodica-Me, I think you need one of these at some point.