There is ongoing confusion about the number of children Mrs Agnes Brown/Browne has/had. It really depends on which version you see/read. In the original RTE Radio version show "Mrs Browne's Boys" (brown with an e) the fifth son is/was Simon, so far unseen in the first 3 series of the 2011 BBC/RTE TV version, had a stutter/stammer. Simon is Rory's twin. [That's 6 kids] This was a more serious radio drama version with the character Pierre/"Fluffy".
Then that radio show story was recycled into the 4 mid-1990s books, "The Mammy", "The Chiseller", "The Granny" & "Sparrow's Trap", and the spin-off "Agnes Browne" movie. Confusingly in that movie there were 7 children - "Frankie" was an additional son. Pierre appeared in this version.
Note - The e was dropped from Browne around this period in the late 90s.
A re-boot of the radio series & books became a stage-show version series, which in turn spawned the 7 part DVD series. Simon was explained away in one of the 7 edition Straight to DVD Series as working nights in a hospital. Frankie has not been included in this version. Known on IMDb as "Mrs Brown's Boys: The Original Series". Originally pretty much self-published for Ireland only, Universal are licensing them.
That version was then re-booted again into the BBC/RTE/Bocpix version. So, what happened to Simon, depends on which version of Mrs B's Boys you watch/read...
In order of versions: The original RTE Radio Show, the 4 novel books, "Agnes Browne" (Angelica Huston film), the stage shows, the Straight to DVD series, the BBC/RTE TV Series, D'Movie.