I’m presently reading Without a Trace: Ireland’s Missing, which is journalist Barry Cummins’s second book on missing persons in Ireland. The first book I talk about on Charley’s recommended books page (which needs to be updated, but that’s neither here nor there). Instead of individual chapters for each MP, he talks about them in a continuous narrative, also covering unidentified bodies, and MPs who were found murdered, etc. The book covers both Northern Ireland and the Republic of Ireland.
A lot of the MPs Cummins talks about in this book were victims of the Troubles, in all probability killed by the Irish Republican Army. The IRA has since then directed authorities to a lot of bodies, but not all of them. The problem is, this book is obviously written for an Irish audience. Cummins doesn’t go into detail about the Troubles, because he assumes the reader already knows plenty about them. But I know jack about the Troubles (the British were acting like jerks and there were people campaigning for Northern Ireland to separate from the UK, and there was a lot of violence — that’s all I know) and I found myself mystified as to just why these particular people were kidnapped in the first place, and what they were “interrogated” about, etc.
The Charley Project Blog 
I believe most of the Disappeared were believed by the IRA to be informants or agents of the British government, which is why they were kidnapped. The Troubles are really difficult to get your head around, because it’s so convoluted and tangled, and the issues going back centuries. There’s a good book called Making Sense of the Troubles which you can get on Amazon (and for Kindle).