Hi Bert,
Happy Easter from Carrickmacross, County Monaghan.
I have located the following information for you:-
Source: RootsIreland:
https://www.rootsireland.ie/ - a subscriptin website
Andrew Crozier of Killeevan Parish married Jane Corbet or Currin Parish on 6 Nov 1832, Church of Ireland, Currin, Co. Monaghan. Celebrant: Edward Collins. Married in Church of Scotshouse. By licence.
Husband Age: full age. Wife Age: not listed.
Andrew and Jane had the following children that are recorded on that website, however there may well be others:
Margaret Jane Crozier
Date of Birth: 3 Jan 1836
Address: Drumswords
Parish/District: Currin, Co. Monaghan
Denomination: Church of Ireland
Father: Andrew Crozier
Mother: Jane Crozier
No. 22. Minister E. Collins.
Elizabeth Crozier
Date of Baptism: 7 Nov 1841
Address: Drumswords, Killeevan Parish
Parish/District: Newbliss, Co. Monaghan
Father: Andrew Crozier
Mother: Jane Crozier
James Richards
George Andrew Crozier
Date of Birth: 19 Dec 1845
Date of Baptism: 28 Dec 1845
Address: Drumswords, Killeevan,
Parish/District: Newbliss, Co. Monaghan
Father: Andrew Crozier
Mother: Jane Crozier
Occupation: Farmer
William W. Deering
Martha Crozier of Drumswords, Killeevan Parish married Robert McCrackin of Killyleg, Aghabog Parish on 19 Sep 1851, Parish of Currin. Martha was the daughter of Andrew Crozier and Robert the son of David McCrackin.The witnesses were Thomas Crozier and James Corbett. Rev. James Bones, Cootehill.
Source: Births, Deaths and Marriages Victoria:
https://www.bdm.vic.gov.au/research-and-family-history/search-your-fami…
Margaret Jane Crosier married James Martin in Victoria in 1865, Reg. No. 101/1865. Victorian marriage certificates list the names of both parents. Bert, if you are trying to follow up on the Martin's, you will need to know who the mother of James Martin was. Martin is a very common surname in this part of the world, without more information the words needle in a haystack come to mind.
Margaret Jane Martin died in 1923, Ballarat, age 87, Reg. No. 13393/1923. Parents recorded as Andrew Crozier and "Eliza Frances".
Source: Ballarat Cemetery:
https://web.ballaratcemeteries.com.au/Deceasedsearch/DeceasedSearch.aspx
Margaret Jane Martin buried 29 Nov 1923 at the Ballarat Cemetery New, Methodist E, Section 3, Row 2, Grave 9.
Tithe Applotment Books 1823 -1937, The National Library of Ireland:
Crozier, Andw, Townland of Drumswords, Parish of Killeevan, Year 1825:
http://titheapplotmentbooks.nationalarchives.ie/reels/tab//004625673/00…
Crozier, Andw, Townland of Drumswords, Parish of Killeevan, Year 1825:
http://titheapplotmentbooks.nationalarchives.ie/reels/tab//004625673/00…
Griffiths Valuation:
http://askaboutireland.ie/griffith-valuation/index.xml?action=doNameSea…
Drumswords Townland, Co. Monaghan:
https://www.townlands.ie/monaghan/dartree/killeevan/newbliss/drumswords/
I have checked the Irish Newspaper Archive for Andrew Crozier, however he was not mentioned, there was also no mention of a mining accident in 1840. Your post does not mention where the mining accident occurred!
As you live in Australia, I would suggest that you apply for a readers card with the National Library of Australia.
https://www.nla.gov.au/getalibrarycard/
This will give you free access to the Irish Newspaper Archive and JSTOR, where you will be able to read the following articles:
The Parish of Killeevan
by J. Gilsenan
Clogher Record
Vol. 4, No. 1/2 (1960/1961), pp. 16-28 (13 pages)
Published By: Clogher Historical Society
A Killeevan Family in Australia
by P. B. Ó Mórdha
Clogher Record
Vol. 14, No. 2 (1992), pp. 134-135 (2 pages)
Published By: Clogher Historical Society
After the Famine: Emigration from Ireland, 1850-1913
by Timothy J. Hatton and Jeffrey G. Williamson
The Journal of Economic History
Vol. 53, No. 3 (Sep., 1993), pp. 575-600 (26 pages)
Published By: Cambridge University Press
The short answer to your question "What led many to migrate to other countries at that time?" - Work, poverty, starvation, high rents, reuniting with other family members. If I find anything else I will post it here.
Maggie McKenna