Civil registration of births, deaths and Catholic marriages in Ireland began 1864 (from 1845 for other marriages) so there will be no marriage certificate or death certificate for Robert Nicholson but Ruth's death should have been registered.
However, there is very little detail on Irish death certificate: name, date and place (townland) of death, age (often incorrect especially in 1800s as informant or even deceased might not know deceased's age), cause of death, name and address of informant (who may or may not have been a child- any person present at death could register the death), date of registration and name of registrar (who may have been the local doctor).
To find marriage, baptisms of children, etc. you need to search for church records (of they exist). Try
www.familysearch.org catalogue to see if they've microfilmed any church records for that period (under place search in library catalogue). If so, you can order the microfilm from your local LDS library for a small fee to be viewed there.
Griffith's Valuation took place in mid 1800s. Here's a link so you can search by name or place:
http://griffiths.askaboutireland.ie/gv4/gv_family_search_form.php
