The really strange thing (that really bugs me) is that whenever Agnes talks about her children, she refers to having six, but only 5 have ever been mentioned in the show. The sixth child has never been mentioned by name though. Even the BBC site says there are six children.
Perhaps this one is being saved for the next series. It took a while for Trevor to make his debut, but since he's so rarely on, you tend to think about the four regular ones. The 1989 cast in the movie 'Agnes Browne' including her, might as well be a different family altogether so I'm not sure Brendan will ever write for all of them ever..