Its possible that two separate families used the same/very similar coat of arms in different parts of the UK and Ireland, so it can sometimes be difficult to be sure. Unfortunately with a stone carving its impossible to tell what colour it is, which can make all the difference!
There seems to have been a family called "Welch" which used the same pattern, but that doesn't mean its definitely that family in this case. You can see a bit about that coat here.
http://homepages.rootsweb.ancestry.com/~walsh/crest.htmlWelch and Walsh is a very common name and represents a large variety of unrelated families with a large array of different coats, but I notice that there was a "Welch" family (as opposed to the more common Walsh) in the general area and are buried in the 1700s in the Hill of Slane graveyard:
http://www.slanehistoricalsociety.com/index.php/headstone-inscriptions/19-hill-of-slaneThe 1659 Census (which doesn't cover the Slane barony) suggests the Welch name was fairly plentiful in the county back then.
Don't think you can say for sure, but hopefully between this and the Roche and other few leads above there might be something to go on!
As for the monument itself, by the look of the mortar work it looks like it was put together in the last 20 years or so, probably using stonework already on site.
