The book is "Maraíodh Seán Sabhat aréir". I think it was written by Cathal O Sandair. I read it about 1965-66, and I might read it differently now with hindsight, but there was no such claim in the book.

He is preented as a person of great integrity who was very committed to restoration of the Irish language as well as to the 32-county republic. He served in the FCA (~=National Guard/TAVR) and rose to the rank of 2nd Lieut. He apparently saw this as a way of learning military skills which he would put into practice in the IRA, and he resigned and joined the IRA, as not to have resigned would in his view have been to break his oath of allegiance sworn when commissioned.

The way he is portrayed in the book is in some senses as a successor to Pádraic Pearse, whom it became fashionable in subsequent decades to denigrate as a "mad queer" (in fact the resemblance even included a shared romantic military incompetence). There may well have been repressed homsexuality in either or both case, but the people making the claim about Seán South were probably just jumping on the revisionist bandwagon.

The Irish teacher who lent the book to various members of my class was careful to emphasise that he was not encouraging us to join the IRA, which surprised me at the time, when we were pretty much all "sneaking regarders". I believe that South was the sort of honest but naive "true believer" who wanted to wage a strictly military war on the British "occupying forces" and that he would have been appalled by the way things turned out in the 1970s and subsequently.