Twiggy first of all can you leave personalities outside of the issue since we only need to deal with legal principles and whether or not people are discordant with you or your family does not really matter. Firstly how do you know you gran died in testate , who has confirmed this ? If this is the case then you will need to list all of her siblings and spouse , detail the type of house / farmland , find out her ownership of the particular house or farmland and then work out who is entitled to what.
In order to check for the deeds you need to contact the local land registry office to see if the land is registered , on the registration you will see a folio number which represents the deeds , this folio number can be presented to your local deeds office (Kings Inns - - Dublin) and for a few pound they will copy you the deeds.
Now it becomes and issue if the land is not registered , you then still go to the local deeds office and ask them to search under the address , name and date of purchase.
On foot of that if your gran had the freehold of the property and no spouse then the property will be left equally to her siblings under in testate however if she left a will then you will have to follow the will unless the terms of the will are invalid.
In this instance you need to inform the son that the property he is staying in or stating that his is the rightful owner is the property of his grans estate and before it can be dispersed, sold or moved on it needs to go through probate and then into administration before any land can be transferred. Unless she did this while living and in fact she may have gifted this to the son and you only way then would be to plea that she did so while be unfit to enter into a contract of sale. It looks like the treatment you have received from the other solicitor is less than pleasant if you are to be believed and in that case you can ask the law society to get involved and/or get an out of town solicitor to deal with him.
To be honest doing the searches is easy and it will clarify who owns the land/house and then you can move forward , if the sibling fails to remove himself from the property or acknowledge that there are many beneficiaries you need will need to serve proceedings on him and do so as soon as possible.