This struck me as a pretty clever concept. An Anglo-Irish lord is getting threatening letters in 1939 and because they suspect the remnants of the IRA may be involved, the famiy see little point in going to the Gardaí, whose sympathies are more likely to be with the IRA, their erstwhile comrades, rather than with the remainder of the old enemy.
It is also quite logical that a murderer who had reason to kill an Anglo-Irish lord might set things up so that it looked like it was politically motivated.
However, quite a bit of the research in the novel seemed...a little haphazard. It seemed as in Koning wasn't really fully aware of how much changed with independence - there is one really weird line when they talk about how the IRA might be to blame in order to further their aims at independence when the book takes place 17 years AFTER independence and it is clear Koning knows this as another character mentions that we got our independence. Equally, there is mention of somebody having shot a "Gardaí office" during 1916, BEFORE Ireland became independent and it doesn't really make any sense for a member of the Irish police force to have been shot in the rebellion against the previous occupiers, since the Gardaí could hardly have been formed at that point and they weren't. It would have been the RIC, the police force that served under the British.
There's also a part where a character talks about serving in World War I and "we haven't always kept out of our neighbours' quarrels," which is an odd thing to say, since...Ireland was under the UK at that time and didn't have any choice about being involved and almost certainly wouldn't have chosen it; there was huge anger in Ireland when it was suggested that conscription be extended here, to the point that it had to be shelved.
And perhaps most oddly, the Blueshirts are described as being Protestant. They were not. They were a quasi-fascist organisation and even argued that Ireland should support Franco because of shared Catholicism. It seemed like the author was confusing the Civil War with the Northern Troubles.
I'm also a little surprised at how convinced everybody is that the murderer is bound to be executed. There were only 3 or 4 executions in Ireland in the 1930s, compared with 11 communted and given the political situation, I would imagine the government would have been wary of executing an Englishman, which could seem politically motivated.
It was a decent murder mystery and the court case was written in such a way that it was hard to tell whether or not the defendent would be acquited (the Defence lawyer beginning a speech to an Irish judge and jury in an Irish court by insulting the Irish Prosecution lawyer and lauding an "English gentleman" really was a bizarre choice).
The fact that the detective was both blind and a foreigner meant he was at a number of disadvantages. He couldn't see what was going on and the gardaí were not likely to accept him as he was a veteran of the army many of them had fought against.