Joint Tenancy and will

I was bequeathed by my bachelor uncle all of his property, there was no specifics however he didn't have a lot, a few bob in the bank and half a house there was no other person mentioned in his will, however the house that he was living in was owned by himself and my mother 50/50. My mother and sibling are adamant that they owned the property in Joint Tenancy, therefore his 50% passes to my mother.
To complicate the whole thing nobody seems to know where the deeds of the property are, how do I go about finding out how they held the property so that I can be sure I'm not being cheated

If the property is registered with the Land Registry, then the nature of the ownership will be on the folio - if they are joint tenants, the ownership will be "A and B are full owners. If on the other hand they are tenants in common, then it will be A is full owner as tenant in common of X shares. B is owner as tenant in common of Y shares"
You can check whether it is registered at the PRAI webiste (you can search online by name, or using the map through the landdirect portal)

If it's not registered in the Land Registry, then it may be registered in the Registry of Deeds - summaries of the deeds (called "memorials") are kept in the Registry of Deeds, and these can be inspected. These are often used for reconstituting title when the original deeds have been lost, and should contain the particulars you are looking for. However, searching the Registry of Deeds is more difficult than Land Registry, and you should ring for advice on how to go about it, or possibly engage the services of a legal searching firm.

Joint Will and Tenancy

I got a copy of the Memorial from the Registry of Deeds, however there was no mention of how the property was held, only that the property was bequeathed to them by their mother. I understand that the deeds are held by the solicitor who drew up my grandmothers will, can I request to see a copy of these as my uncle name was on these deeds and I am now executor of his will. If there is no mention on the deeds of how the property was held ie Tenants in Common or Joint Tenants which takes precedence

That sounds like it's a joint tenancy. In the absence of "words of severance" apportioning shares, the people take as tenants in common.
However, to be sure, you'd need to see the original deed. You should ask the solicitor.

@wicklowlad - just to add that for there to be a joint tenancy - the four unities need to be present - possession - interest in land needs to be identical - title acquired at same time (eg by will) - time: needs to vested at same time. Adding what @j26 says is important.

The Solicitor should know.