My personal experience is net of tax, as in the amount physically received into a bank account.
In the past, when income levels for means tests have been altered, if a person has qualified under one set of guidelines for a particular payment, they continue to qualify under those guidelines, & only new applicants are affected by policy changes.
All things being equal, & assuming that your mother declared all income at the time she was means tested for Fuel Allowance, & her circumstances have not changed since, I would assume that her entitlement would not change either.
But, as Black Sheep has pointed out: Anything is possible in the future, so my advice is to wait & see what transpires on budget day. Presumably things will then be crystal clear........
Should it transpire that the automatic entitlement to the Household Benefits Package for over 70's will after the budget be means tested (like the Medical Card automatic entitlement in last years budget), who knows what the income threshold might be. Either way net income would be what's assessed as means in my opinion.
Simply put, if someone has been granted a Fuel Allowance, & their circumstances have not changed since, the chances are that any changes in means test guidelines will not affect them.