Compulsory registration of land is a very recent thing in Ireland. It only started around 1990. Prior to that, if land was sold, a deed would be drawn up and signed by the relevant parties but there was no requirement to register either the transaction or the new ownership anywhere. (The new owner normally acquired the sale deed unless, as mentioned previously, it was lodged as security for a mortgage or loan).
Some deeds were registered with the Registry of Deeds in Dublin. This was an optional service in case important documents were lost. They made a summary (called a memorial) and entered it into their records. There was a fee for this and so many didn’t bother. Their records start in 1709 and can be found on the LDS site. Search under the townland.
If the Patrick Murphy I found in Griffiths is your family, then he didn’t own the farm (at least not in 1854 anyway). He rented from Plunkett. However he could still sell. What he would be selling was all the stock, seeds, crops planted in fields, agricultural implements etc on the farm, as well as the unexpired portion of the lease. So if the family legend is that the farm was sold, that may be what was actually sold. Such a sale would normally be by auction and might have been advertised in a local paper but no state record would be kept of the transaction.
If you are trying to find out when it was sold, then the Valuation revision records in the Valuation Office in Dublin should record that. The Griffiths clerks revisited properties every 2 or 3 years and recorded changes in tenant, land size, extra buildings, dilapidation etc and revalued accordingly. Their records books (sometimes called the cancelled books) record the year of the change (though they are often out by a few years). Those records go up to at least the 1930s. They are not on-line, except those for Northern Ireland, so for Co. Monaghan you have to contact the Valuation office and ask for a search.
https://www.valoff.ie/en/archive-research/
Around 1903 Wyndham’s Land Act allowed most people to buy their land if they wanted, with a Government Mortgage. Most farmers took advantage of that. Where they did, the relevant entry in the Valuation Revision books is normally noted “L.A.P” in purple for the relevant year. This indicates a Land Act Purchase. Perhaps your family did so? I have attached apage of records for a townland in Co. Antrim in the 1890s to give you an idea of what the Valuation revision records look like. The clerks used a different coloured ink each year and the year of any change is normally recorded in the right hand column.
