It's explained in more detail in the comic, and a bit too subltly in the movie, but in this world racism is in full-force. If you aren't English (Scottish is sometimes okay), then you are scum and need to die. Irish, Welsh, American, African, Indian, anything that isn't white English is evil and cancerous and must be destroyed.
When they bring up that Creedy's mother was Irish, it's a way of saying 'are you trying to undermine this?' Was he just looking into this stuff because of a personal connection, or was he being entirely logical and looking at the facts?
On a side note, in the comics there's a TV show called 'Storm Saxon' about a stalwart square-jawed British action-hero who fights off the tyranny of the blacks, it's a very racism-promoting show and they actually included it briefly in the movie, when Evey is going to her work the guard at the front desk is watching 'Storm Saxon' on his TV, although they don't name it. He just mentions the name of one of the characters, 'Laser Lass'. This is what I mean about it being a bit too subtle, in the comics racism is normal in that world, in the movie it's the same but not as clearly stated.

St. Marys was a school in London, how could the virus there have killed a lot of people in Ireland?

Uhh, the St. Mary's Virus killed lots of people there. Really?