Sorry, I was wrong. There is coal in Northern Ireland and some great efforts were made to exploit it. I was right in the fact that there was a big export trade in coal from the west coast of Scotland and Cumberland to Ireland.

"In east Tyrone small amounts of coal had been dug out of the ground from the middle of the seventeeth century, but the sinking of deep shafts in the 1720s revealed deposits on a scale not yet found anywhere in Ireland."

(A History of Ulsster : Jonathan Bardon [The Blackstaff Press 1992] ISBN 0-85640-466-7 p183)

"During the 1654 Civil Survey it was noted that coal was being mined at Tullyniskan in east Tyrone. More coal deposits were found nearby at Drumglass in 1692 and these were being exploited by somewhat crude techniques for a Dublin consortium soon afterwards. Then in 1723 Francis Seymour, an entrepreneur, leased land from the archbishop of Armagh and began mining by more sophisticated methods at Brackaville, soon to be known as Coalisland. Seymour sank a shaft 156 feet deep at his 'Engine Pit' and a few years later a 'cut' was begun to connect Coalisland to the Blackwater to take the coal to Lough Neagh."
(ibid p203)

"The population of Dublin had increased sevenfold since 1660 and it was fast becoming the second city of the empire - a mushroom growth not possible without massive importation of British coal each year. Colliers had to face prevailing westerlies, and supplies of coal were expensive and unreliable. With the optimistic prospect that the Tyrone coalfield could supply the capital's needs, the Newry Navigation was begun in 1731. .... the canal was completed in 1742 and on 28 March in the same year the 'Cope' and the 'Boulter' of Lough Neagh sailed into Dublin with cargoes of Tyrone coal.

...

The Tyrone coalfield never achieved what was expeted of it. Severe faulting made mining difficult -"
(ibid p204)

But by the early 1800's

"Ten one-hundred-ton vessels plied the North Channel to bring over the six thousand tons of Cumbrian and Ayr coal needed each year to raise the steam in the [Belfast] mills"
(ibid p 259)

There was coal in Ireland and mighty attempts were made to exploit it in the mid 1700s (including the construction of an impressive ship canal). However due to the difficulties with the seams it was the easier worked Ayrshire and Cumbrian coal that eventually supplied Ireland.

In the 1920s a big effort was made to revive the Tyrone coalfields and after sinking a vast amount of government money into the project the project was abandoned for similar reasons to those of the late 1700s.

If the Caldwell family in Ireland had any conection with the coalfieds then Coaliston is where they would have been.

Griffiths Valuations do not seem to recognise any propertied Caldwells in east Tyrone (although Caldwells may well have been there).

My impression is that the Caldwells in the Ayrshire coalmining industry were very much a local phenomenon and had come to work there off local Ayrshire farms.

Many Irish came to work in the Scottish coalmining industry as it was opened up in the 1800s. I doubt of these were skilled miners before they arrived. As there had been Caldwells in Ireland for generations by this time it is very likely that some Caldwells in the industry may have been Irish immigrants.

This brings me to another point: to this date I have not found any other Caldwell families in Scottish coalmining that were not of our family. I would have thought that there might be many Caldwell families in this large industry but this, so far, has not proved to be the case.