The difference between slavery and pre-social reforms factory workers is not very big. De jure the workers might be free but de facto they were worse off.
And who said than slavery would prevent any industrial development?
BTW some sort of slavery still exists today: prison farms (and similiar institutions) is atemporary form of slavery, with the state as slaver (especially if the state leases the prisoners to others).
When a factory worker can be beaten to death for any reason or none, or raped without consequences, or have his/her family split up, or isn't able to marry (in any recognized way), or otherwise treated as property with no rights or protections granted by the law...
Sure, Irishmen in some conditions might have been treated as expendable and cheap where as slaves were valuable, but I'd rather be an Irishman working a 12 hour day in a factory than a black slave doing the same or a black slave in most conditions we do see them in.
The fact the I
can legally better my lot is a substantial point in the favor of freedom. Both by social reforms, and in terms of my personal lot - if I save enough money, I can do something with it.
Working for George Pullman (picked as the best example of wage slavery's level of control over the workers) is hardly a desirable scenario, but I'd put it well above plantation slavery.
As for who said slavery hinders all industrial development: I believe Serenissima has been advancing something along those lines.