You will find some records for a Robert Mercer (without Minchin) on the pay site www.findmypast.ie
. You can buy credits and I believe it is not too expensive.
Slater's directory 1846 shows a Robert Mercer occupation Baker at 8 Dublin St Clonmel
There is a death for a Robert Mercer Clonmel registration district 1864 volume 14 page 363. You can get the death cert by downloading an application form from www.groireland.ie
- cost is less if you ask for a photocopy of the entry in the register -it gives the same information as a full cert and costs €4.
also there is a possible marriage for a Robert Mercer Clonmel registration District in 1852 volume 4 page 8 with a bride possibly Susan Smith. The reference is to the page in the register and there can be up to 4 couples on the page so this is not a certainty but is highly probable.
Hope this is of help.