I can't find any reference to Richard Williams and I haven't heard of Mountervelin.
There is a record of the birth of Eliza Anne McWILLIAMS on 14/05/1813 in Ballyorr, GLENDERMOTT Derry.
She was baptised 06/06/1813 in the Church of Ireland and her parents were Richard McWilliams and Sarah Anne McKey .
There is also an Andrew McWilliams b.13/10/1818 and a William McWilliams b.13/03/1823...parents Richard and Sarah (no maiden name) in Limavady.
There is a Joseph McWilliams b.01/02/1826 in Tamlaght Finlagan...parents Richard and Sarah Kane