This would presumably explain the three red roses on the family coat of arms (see icon); but I wouldn't mind being able to find some slightly better proof of the existence of Sir Maurice. I have a sneaking suspicion that this was all invented to give a respectable background to my Elizabethan namesake.
Sir Maurice Whyte, who served in France under Henry IV and Henry V where at the siege of Rouen, with the Prior of Kilmainham, he led 2,000 Irish, and later made Governor of Montaire under Henry VI. He was called "The Lancastrian", having served under three kings of the House of Lancaster.
Well, some of this is easy enough to put dates to. Since Henry V reigned for less than ten years (1413-1422) it is not difficult to imagine a military career that would involve serving under both his father and his son. The siege of Rouen lasted from July 1418 to January 1419, and the presence of the Irish soldiers is well attested, as is the role of Thomas Butler, the Prior of Kilmainham; though British and Irish estimates of the troop strength under his command are more like 500-700, a French source describes "eight thousand Irish savages" as being part of the English forces. There is no mention of Maurice the Lancastrian in any on-line sources, but probably I can get into this the next time I am in a decent university library.
The idea of Maurice having been "Governor of Montaire" needs a bit more exploration. I can't find any Montaire in France; much more likely the story refers to Montoire-sur-le-Loir, now more famous as being the place where Pétain and Hitler agreed on French collaboration with the Nazis in 1940, but which was certainly on the contested border between English and French zones of control at that phase of the Hundred Years' War. I can't find on-line references to any particular local set-up there in the 1420s; other possibilities are Montoir-de-Bretagne in Brittany, and the Château de la Montoire near Calais (though this last appears to have been in French hands between 1410 and 1488). Again, I shall just have to wait until the next time I am in a decent university library.
Would any of you have easy access to the journal The Irish Sword? Only I see that it had a very short article (pages 62-63 of vol 2, 1953) by Richard Hayes on "Irish soldiers at the siege of Rouen, 1418-19".