There were a good number of Costellos in Clonoulty Tipperary (probably more than 100 I have researched)? I have seen the name in other counties, mostly in the west of Ireland and of course Dublin and Wexford, some in Kilkenny too. A general search in the parish registers will show there are more than 9,000 baptisms of Costel* in Ireland. You have to remember that maybe as few as 50% of people registered their births before State registration. Not everyone was Catholic, it was seen as irrelevant by many and the records could be used to track people who disagreed with English rule.
I have made the assumption, rightly or wrongly, to put two historical things together; that there was emigration of "Italian" mine workers to other places in the 1700s; Tipperary in the 1700s had Silver and Copper mines - a little like Cornwall in the uK? Small by todays standards but at the time a valuable commodity, on the doorstep, even though they did not need copper cables in those days! :)
I have DNA connections with many Costellos and you will see on the Clonoulty parish here on IrelandXO the main Costelo of interest married to Walter Crowe. I have a DNA link to the descendants of this family too. This tree also appears on www.crowegenealogy.net. So, I would recommend a DNA test because, despite what people might say, it is only useful up to three, maybe four, generations back in time. From this time backwards it will be like finding a needle in a haystack, you need a family tree with lots of names (dozens if not hundreds) and a lot of corroborating evidence (like locations) to go with a DNA test - ask any genelaogist about the reality of DNA tests to find family beyond second cousins? This is normally a problem but especially difficult for people trying to trace back into Ireland - I reckon anyone doing Irish genealogy has to be worth their salt because it can be difficult and you will need some patience?
Do not be put off either, keep chipping away and I am sure you will find your answer. Please let me know if I can help further?
Seamus