I had a look at the Dinans, just because I've never done so before:
John Barry Henry (some sources omit the 'Henry', but it's on company records etc) Dinan 1939- of Chalkpit House, Knotty Green, Beaconsfield, Bucks., management consultant, Capt., Irish Guards m. 1st, 1961, Amanda Mildred, dau. of Sir Lacey Eric Vincent, 2nd Bt (issue 2 s.); seems to have m. 2nd, 1980, Wendolen Maw (this being, on investigation, a Wendolen de Beer [1943-], who m. 1st, 1967, Jean Marie Daniel Couve, a banker, and 2nd, 1980, Carlyle E. Maw, the only candidate for whom appearing to be Carlyle Elwood Maw [1903-1987], an attorney who served as U.S. Under Secretary of State for International Security Affairs 1974-6, having been Legal Adviser of the Department of State 1973-4). The 1986 birth of their son Richard gives mother's name as 'Dinan or Curzon', but the marriage seems to have taken place in 1988.
J. B. H. Dinan s. of Dermot Anthony Dinan (as above), of Egerton Place, S.W.3., formerly of 29, Malvern Court, S.W.7, formerly of 19, Talbot Street, Dublin, a timber importer (inc. sole selling agent of plywood manufactured by A. Ahlstrom Osakeyhtio and Aktiebolaget Kaukas Fabrik, Finland[!]) Pamela Joan (née Kennedy) m. 2nd, 1957, the 2nd Baron Windlesham, and 3rd, 1966, William Marsden Elverston-Trickett, pilot officer, R.A.F. (presumably the W. M. 'Bill' Elverston-Trickett, solicitor, featured in James Comyn's 'Summing it Up: Memoirs of an Irishman at the English bar', where it's mentioned he assumed the 'Trickett' [thinking it somewhat unfortunate a name for a lawyer!] as the condition of a legacy).
D. A. Dinan s. of John Dinan (b. c. 1859/60 per 1901 and 1911 Irish census records), of Knockeven, Rushbrooke, co. Cork, formerly of Castlemartyr, co. Cork (an 800-acre estate with castle etc), formerly of Queenstown (Cobh), co. Cork, timber importer, chairman and director, Eustace & Co. Ltd timber merchants, Francis Spaight & Co. Ltd, etc etc., J.P., and his second wife, Kathleen (b. c. 1877); his first wife's identity I have been unable to ascertain; John was a widower in the 1901 census, and married again by 1911. John Dinan had five sons by his first marriage: the eldest son, William (b. c. 1885 [d. 1981?]) was of Coolgrena House, Cobh, educ. Pembroke Coll., Oxon. (M.A.), and was a director of the companies with which his father was involved; a Pembroke College online source makes reference to a William Dinan having died in 1981 aged 98, having also previously been a student of Mount St Mary's Coll., Chesterfield; this is presumably the same individual. The second son, John A. Dinan (b. c. 1886), was an American Consular agent at Limerick, where he lived at Lyndhurst, and director of a timber, iron and hardware company there. He too was a director of the companies with which his father and brother were involved. The three younger sons were all killed in the First World War: Frederick Charles (1889-1918) was a Major in the 1st Essex Regiment, late Royal Field Artillery; George Albert (1892-1916), educated at St Augustine's Benedictine Coll., Ramsgate, Univ. Coll., Cork, and Guy's Hospital, London, was a 2nd Lt in the Royal Dublin Fusiliers, and Francis Arthur (1894-1917), educated at St Augustine's Benedictine Coll., Ramsgate, was a 2nd Lt in the Royal Field Artillery.
The 1901 census shows John Dinan's mother, Ellen (b. c. 1817) in her son's household, listed as a widow; her son was born at Cork city, which is where they presumably lived.
A connection to the extinct Barons Dinan is not impossible (if very unlikely), but MacLysaght in 'Surnames of Ireland', states as follows: '(O) Dinan, Dynan- Ó Daghnáin. Now chiefly Cos. Clare and Cork. Quite distinct from Dinneen though in Cork Dynan has sometimes been changed to Dinneen.' Michael C. O'Laughlin, in 'Families of County Cork' vol. 4, gives: 'DINAN- O'Dinan/ O Dyeyneene/ Dinane/ Dinneen: Dinan, Dineen and Dinneen are names given in Cork in the Birth index of 1890. Dinane is the spelling given to the family name in the census of 1659. The O'Dineens are given as hereditary historians to the MacCarthys. Some references give the O'Dinane family to the Hy Nial tribe settled in Cork, others give the family of the Corca Laoidhe in Co. Cork or of the Eoganacht Aine territory in Limerick. Note the territory of Tuath O nDuinnin in north Cork on the Limerick border. In this vicinity we also find Ballydineen townland. The family name can be found in Cork in the 1659 census, and is likely recorded also as Dinane and Dyeyneene, in that document.'
The overall impression is of an indigenous Irish descent, rather than that of the Barons Dinan, viz. (per Burke's Dormant, Abeyant etc Peerages, 1866) : 'The surname of Dinan appears to have been first adopted by Fouke, one of the knights of Roger de Montgomery, Earl of Shrewsbury, upon whom that nobleman conferred the castle which he had erected at Dinan (now called Ludlow), and he was thence designated Sir Fouke de Dinan.'