Seamus' father, James, came over to Ireland with "the boys from Bristol" when Richard de Claire (Strongbow) started his forays into Ireland beginning in 1169. As a lesser son of a lesser noble his only chance for an inheritance of his own was to make one. He was neither particularly good nor particularly well liked by the rest of the forces. He did help them in aiding various Irish factions against various other factions. But whereas most of the other Normans ended up with bits "liberated" from factional enemies, he never got quite so lucky.
Seamus' mother, Grainia, on the other hand, was the inheritor of the estate of a minor Irish chieftain in the Glenageary area. It was all very clear to her that the "helpful" Normans were really just out for land of their own. Their professional army and the defensive architecture made it very clear that they intended to keep what they "liberated". She selected James and, effectively, proposed to him. He seemed dim enough that she could still manage the lands appropriately without undue interference, but being then a Norman wife she would then be immune to any raiding or appropriation by the new forces.
From James's point of view nothing could be more perfect. Without having to do any real work he could gain an enthusiastic wife, with lands and cattle. The fact that she wished to be married under Irish Brehon law, rather than English Common Law seemed quite a minor point.
So, in time, Grainia ingratiated herself with the Normans. Being astute at Irish politics and an astute observer she aided various of the rising Normans by suggesting various factional enemies of her extended family that needed "liberating" and provided the intelligence to make it easy. Seamus was conceived and born for appearances. Eventually the initial settling in period of the Normans had passed and their politics and factions became intermingled with local ones. From Grainia's point of view James had passed his usefulness, so she divorced him, under the provisions of Irish law. None of the other Normans raised any objections as she had been far more use to them than he. Under the Brehon system a divorce is settled based on how each half of what was brought into the marriage prospered. Since James brought essentially nothing, he was sent on his way with nothing. The last Seamus knew of him was he was going off on Crusade with King Richard.
So Seamus was raised largely by his mother. Or, rather, his mother's household. He has a great respect for Irish history and tradition, particularly the stories of strong heroic women. Yet he is also instilled with Norman heroic idealism. The high point of his life was when King John visited in 1212. He was present when he went to mass in Christchurch Cathedral and saw him from a distance.
Not long afterwards Seamus dies. He believed he was heroically defending innocents in a battle against injustice. In reality it is just another pointless Irish faction fight. He now finds himself mixing with others from many other times, locations and traditions in the modern SCA. It isn't quite the afterlife he had expected but he assumes he just didn't pay close enough attention in Church.