I had a relative under police observation in the same era but in Co. Tipp. – he was described in a later report as a ‘well known Fenian’!
France generally has very good records, the main issue is where to look.. Napoleon divided the country into administrative regions, called Departements (similar to Ireland’s ‘counties’). The difference between Ireland and France is that there is no ‘GRO’ in France and each Departement holds its own records, at the Departemental HQ. known as the Prefecture. The Departements are numbered in alphabetic order, so e.g. 01 is Ain, 02 is Aisne, 06 is Alpes-Maretimes etc., and Pau is in the Pyrénées-Atlantiques which is 64. A great source is the ‘Livret de Famille’ which is somewhat like a ‘family passport’ in which all family BMD records are entered – it exists to this day.
Most of the Departements have put/are putting all their records online. There also are collections of scanned military records and as your Mr. O’Hea was in the army for years he would have had a service record and military pension, so there would be records of that. I’d suggest you have a look at this site http://www.geneafrance.org/rubrique.php?page=numeris#dep64 (there is a translation button on it). I found it a very useful portal in the past. Come back if you are unsuccessful – I have links (on a back-up HD) for military records/pensions that I used for researching a Napoleonic army soldier for an IGRS talk (father-in-law of JFFuller).