Have you access to a copy of the “The Treston Family” edited and produced by Dennis Treston of London in conjunction with the Mayo based Noel Treston. This provides the date of death of Rose Fullan Treston as 21 Aug 1877, but only gives an approx date of death for her husband Hubert as c.1870. Dennis is a determined researcher so this information may not be easily available on normal records. If you find it please share.
For information Deansgrange cemetary was first opened in 1864 or 1865. prior to that RC burials in Blackrock were in Kill O' The Grange Cemetery,( closed 1865) or at Tully burial ground both closeby.
seeMemorial Inscriptions of Dun Laoghaire-Rathdown, Co. Dublin, Ireland" Volume 1 - ISBN 1 898471 66 5 Compiled by Barry O'Connor & Brian Smithhttp://www.dun-laoghaire.com/genealogy/publications.html
You should also note that Hubert’s 9 childern had their birth registered as Treston(5), Tristan(2). and Triston(2) and his mariage was registered as Tristan. Some later generations appear as Tresson. Hubert’s burial could be under any spelling? and as he appears to have owned property at 1-4 Brookfield Place,Blackrock his death should have been registered. Hubert is believed to be the son of either Cyrl or Peter Treston born c.1785 in Mayo.