quote from john kirkpatrick
The chromatic style of playing seems quite popular among Irish musicians, who like melodeons in D and D, although of course they don't fit in any bass. When I once asked an Irish box player what be did with his left hand, he said "I don't know, I just look the other way."
this is not entirely correct.
[john kirkpatrick generalising from one comment]
irish musicians used c csharp, d dsharp, bc, c sharp d and even occasionally d g.[ eg rose murphy]
the c sharp d has basses that are correct when you are playing in d major
and as i previously mentioned if you want to play in c major as some musicians do, the double ray, bc sometimes has the correct basses.
it is also very possible to play right hand harmony, on the c sharp d, when you are playing in d major it is similar to playing a one row in d major