Hi Bryony! That's such a fascinating family connection!
For tracing the Lefroy family further, I'd recommend taking a look at the
National Archives of Ireland, if you haven't already, as these hold a range of civil, church, census and other historical records. As the family had connections to Dublin, the
Dublin City Library and Archive may also be worth contacting. It also sounds like the family vault you mention may be at
Mount Jerome Cemetery. The relevant cemetery records could therefore potentially provide further names and family links.
I hope this gives you some ideas to continue the search!