I wrote up my family history (grandparents, great-, and great-great-) focusing on each individual in turn. I think it worked reasonably well, because there is no real narrative ark in most family history, so an essay-style text is hard to write in an engaging manner. The challenge is to avoid repetition when referring to the same event from two different perspectives. But you may be a better writer than me. If your experience is anything like mine, you'll find the appreciation from relatives will not pay back for the effort involved. It is only worth doing if you are enjoying it yourself.
You'll have to decide what output you want. If it is a family tree, an online site is worthwhile, because even if you have the information already, the online software will populate the tree automatically with identified census data. If you have the information already, the data should be easy to find online (although I'm not sure about Irish records).
If your records are really thorough, and you want a written text, rather than just a tree, you will probably benefit from online resources. Assuming your relatives are in the UK, the best ones (apart from the basic census data which is accessible from Ancestry etc) will probably be UK wills (probatesearch.service.gov.uk) which is a pay-per-order for each will and the newspaper archive (britishnewspaperarchive.co.uk) which is a monthly subscription service. Because your mother's work is 15 years old, she probably didn't have access to the 1911 census data and certainly not the 1939 data, so there is additional information to access.
In my family history there were several candidates for the relationship section of mumsnet (and a couple who did indeed LTB, even in the 1860's). My great-great-grandfather was prosecuted for abandonment of his family, and I found an account of the court proceedings in a local newspaper (online) which was fantastic. You may or may not find something in the newspaper archive, but it is worth trying, because it is a real eureka moment when you do. Occasionally the national archives can turn up something (nationalarchives.gov.uk) although they are much less searchable than other resources. (In the national archives I found the operations report for the bombing raid in which my uncle died during WW2, which was another poignant moment. My father was very moved when I gave him a copy.)
Hope this helps.