According to SW website, for claims made in 2009 or 2010, Illness Benefit is paid at €88.10 pw for persons whose average earnings are €150pw or less.
The €153pw rate for earnings between €125 and €150pw only applied to claims made in 2008.
If it is more advantageous for your father to claim for your mother as a dependant than for her to claim in her own right, they should contact Illness Benefit Section and ask about how to do that. Your mother should continue to send in certificates, so that her PRSI record is kept up to date.