Peter,
Money.
Agricultural labourers from Ireland, Norfolk, Suffolk, etc, flocked to pits in Northumberland & Durham, and elsewhere in the 1800s.
Not because they wanted to be coal miners, but because they had to find an alternative to "land" work
( because of failed harvests etc, leading to evictions from "tied" accommodation) to put food on the table.
Coal mines were very dangerous, but so to were other elements of the Industrial Revolution. e.g. shipbuilding, ralways, bridge-building, chemical industry, etc.
The Irish, and others, would be faced with an ongoing dilemma.... " Would we rather be starving in beautiful Ireland or eating off our table here in Grottsville Colliery ? "
Few went back.
Michael Dixon,
Blyth, Northumberland