Many thanks for your posting. Sorry to have to inform you that (with the greatest respect to your correspondent from Offaly) that there is close to zero possibility of his story of the origin of the Guinan family being true.
Taking it point by point ..he states that
1. Gunning is anglo-saxon name settled in Ireland in the 1600s
The Gunning name in Ireland is derived from the native celtic name
2. Guinan is a derivative of Gunning
3. Gunnings were probably knights and noblemen who were granted lands in Offaly by the Fitzgeralds
1. Gunnning is not an Anglo-Saxon name - rather it is derived from the celtic name O'Cuning or O'Conuing which is linked with a small barony in Limerick (O'Cuning of Crioch) thus predating the Anglo-Saxon era by many hundreds of years.
Search for this book in google books and look up Gunning:
The book of Irish families, great & small
By Michael C. O'Laughlin
Edition: 3, illustrated
Published by Irish Roots Cafe, 2002
2. Guinan (and my name Guinane) both come from a completely different root - O'Cuineáin/O'Cuinneáin and are so not related in any way with Gunning. This origin of the Guinane name is widely documented in many authorative sources such as MacLysaghts Irish Surnames.
3. Gunnings were Anglo-Saxon noblemen
I have done no investigation of this family as it is a separate tribe to mine. However I will offer the opinion that there is no possibility that a noble anglo clan would have settled in Ireland in the 1600s and changed their name to include the Irish form of O' as in O'Cuning. It would be a relatively simple matter for anyone to lookup the english nobility records and land grants to prove me wrong as these records were carefully preserved in England.
The theory that I am exploring is essentially that the Guinane/Guinan/Kinnanes are river people originating in the Kildysert parish of County Clare and specifically on the Shannon/Fergus river islands who migrated along the Shannon River to Limerick/Tipperary/Galway and even Offaly. This is a work in progress. Maybe by the end of the Summer I will have put together the definitive documented proof (or otherwise). But what I can tell you is that the evidence does exist and so it is a matter of putting it together. I hope to build a picture of the Guinane population at different points in time such that as the clock is 'reversed' you will see the contraction of the distribution to an ever smaller and denser footprint.