[Do most players of Irish music use both rows or do some use only one on a double row accordion?]
everybody who plays two-row box uses both rows at least a LITTLE. even if you stick to keys that play "on the row" of your name row, you're still gonna take a dip to the second row a tad to get accidentals now and again. that would be the bare min. [otherwise, just get a one-row. then you won't HAVE the accidentals and you'll have to do creative setting-adjustment to get around it.]
from using your second row a LITTLE, there's the next level, which is, you still stick to playing in keys located mainly on your home row, but in addition to dipping down to the second when necessary for accidentals, you ALSO now and then use the second row when NOT necessary, to "smooth" certain passages, get longer phrases without doing a bellows switch. contrary to popular stereotype, some supposed "on the row" players do this quite a bit even when playing in the name key of their home row (like, playing in D on a c#/d). these days, half the c#/d players out there sound pretty darn smooth even when playing in D....in any case, how much you avail yourself of these "smoothing opps" is a stylistic choice. you can play in D on a D row of a C#/D pretty much always yanking it back and forth like a one-row, or you can smooooooth it here and there. i smooooooooth all the time when playing D tunes in C on the C row of my b/c, because i'm a smoooooooothie. i want my phrasing to sound like conor tully & paddy carty, not like a one-row. but it's a choice that's totally up to you. for polkas, you can send mobs of people into mass frenzy by never using the second row and always yanking it back and forth "on the row".......
then, the NEXT level up is, you start playing in keys that aren't mostly "on the row." keys that use both rows, so by definition, you're playing smoother. or, keys that go almost ALL in one direction, so you use your air button a lot. like, eminor or aMajor on b/c. somewhere i read that mairtin o'connor commented that he switched from b/c to c#/d because he was "sick of playing it all on the draw," or something like that. well, he must have meant, for certain keys. because c#/d has the exact same # of keys you'd play "all on the draw" as b/c does. they're just different keys.
so it's your choice---depends on what KEYS you choice to play in, and then also depends how much/little you want to do optional alternative fingering using both rows to get a smoother versus back-n-forth-ier sound....