What's the biography of Drennan that you're going to read called? I've been wanting to read one on him and it sounds promising
I am afraid that the biography is only available in German:
Lahme, Jörg: William Drennan und der Kampf um die irische Unabhängigkeit. Eine politische Biographie, Göttingen 2012.
The title translates to William Drennan and the Fight for Irish Independence. A Political Biography, and I have read the first few chapters and am so far quite pleased with the level of detail the author affords to setting the scene of Drennan's familial ties and early years.
I was in luck and got it (contrary to the price on the publisher's website) brand new for about 3-5 Euro from a discount bookseller specialising in liquidation/surplus stock sometime early last year, I think.
If I recall correctly, Lahme's biography of Brennan was not initially a book project (it's his doctoral thesis, in fact) in that he never intended to publish it, and I would say that that shows a little bit in the structure, which however is not a point of critique.
Sadly, there seems not to be enough of a market for the history of 1798 and in particular its lesser-known participants (everyone knows Tone and FitzGerald; how many people, alas, know Drennan?), which explains why Lahme's biography of Drennan has so far not been translated, which is a shame, seeing as Lahme appears to have done a lot of legwork digging for personal documents by and relating to Drennan in several archives, that I am sure have never been referenced before.
In case you are interested, I could try and post a few quotes from the book in the future (translated of course).