1st things 1st. Get them to make an application immediately.
It doesn't matter if they don't have all the documentation required, this can be supplied later.
As for back dating claims, according to the website
" Late applications
Please apply for your pension on time. If you make a late application you may lose some payment. However, we may backdate the award of the pension for up to 6 months before the date you applied if you have a valid reason for the delay.
In certain exceptional cases we may backdate the pension for periods greater than 6 months as follows:
you failed to apply in time because of incorrect information given to you by our staff,
you were incapacitated by illness or infirmity,
you experience a "force majeure" - extreme events or actions that prevented you applying in time, or
you are currently having financial problems.
In all cases where we consider making a backdated payment beyond 6 months, we must also be satisfied that your means entitled to the payment for that time.
If you think that you may qualify for a backdated payment under any of the circumstances outlined above, please send us the details and any supporting documents."
http://www.welfare.ie/publications/sw16.html#9
Murt