Just a few random thoughts.
Aunty cannot be fairly said to be excluding the girls.
She is just not including them.
The girls would not have a definitive or automatic right to inherit.
The will they have accessed might not be the
last will and testament ! BTW there is no particular right for any potential beneficiary to see the will or to know it's contents until the Grim Reaper has arrived.
The girls might well be able to contest the will under S.117 as referred to by mf1.
The core concept of S. 117 is that a court may order an effective readjustment of the will.
Here is what subsections 1 & 2 provide specifically ;
"(1) Where, on application by or on behalf of a child of a testator, the court is of opinion that the testator has failed in his moral duty to make proper provision for the child in accordance with his means, whether by his will or otherwise, the court may order that such provision shall be made for the child out of the estate as the court thinks just.
(2) The court shall consider the application from the point of view of a prudent and just parent, taking into account the position of each of the children of the testator and any other circumstances which the court may consider of assistance in arriving at a decision that will be as fair as possible to the child to whom the application relates and to the other children."
LINK to S.117 in full
http://www.irishstatutebook.ie/eli/1965/act/27/section/117/enacted/en/html#sec117
The girls would have to initiate the proceedings and make out the case.
That would be a potentially expensive exercise whether they win or lose.
Any orders on costs are at the discretion of the trial judge.
Tax is not my thing but I would wonder if this is a tax efficient will. Much depends on the specific details of the estate which seems to be of substance. The son who receives the entire estate may find himself liable for some capital taxes even though he would be in category A for exemption threshold purposes. If all four children received equal inheritances the risks of tax liabilities might be lower. However, all of this depends on the specifics of the actual estate.
A delicate hint to Aunty about the importance of reviewing wills periodically, especially with a solicitor, might help.
The solicitor might well point out to Aunty the potential trouble she could leave given the present legacies.
Any potential beneficiary would probably be the wrong one to drop the hint for fear of being accused of exercising undue influence
If the last will was drafted and executed by a solicitor it is probably valid. If by any chance it was a DIY job and it is defective it may well not be admitted to probate in which case happy days for the girls
Generally, these are unfortunate situations which can cause terrible bitterness within families if wills are drawn up in particular ways.