Hello and welcome to RootsChat. Your enquiry seems to have been overlooked.
There's any amount of stuff on the Irish War of Independence period. If you live in Britain your public library should have books on the topic.
A recent book is "Atlas of the Irish Revolution". See atlasirishrevolution.ie
Some websites.
The Irish War of Independence - A Brief Overview
www.theirishstory.com/2012/09/18/the-irish-war-of-independence-a-brief-overview/#Wx1R8fZFyM9History Hub - connecting past and present (podcasts)
historyhub.ie/theirishrevolution
Welcome to Century Ireland - RTE (online historical newspaper)
https://www.rte.ie/centuryirelandRTE is the state broadcasting corporation.
BBC News/History/1917-20: The road to partition
The Irish Revolution 1911-1923 Oxford Bibliographies
www.oxfordbibliographies.com/view/document/obo-9780199791279/obo-9780199791279-0110.xmlThis site has brief reviews of some books about the period.
If you want a very brief introduction look up Irish War of Independence and/or Irish Revolutionary Period in Wikipedia.
Googling those terms will lead to more sources but beware bias of writers/contributors.
Other people may have more suggestions.
You could post a specific enquiry on the military history board for information about your GF's service in the Scots Guards in Ireland.