What a gift of a surname!
I’m not sure what websites you’ve already looked at for traces, so some of these may be quite basic for you.
I’m sure you’ve already looked at the IGRS offerings online here in the Resources section.
John Grenham’s website shows a bit of name distribution and an alternate spelling of Lavinder.
http://www.irishgenealogy.ie for all historic birth/death/marriage registers (with data protection limits in place). I just did a broad search and it returned 234 Lavender records. Lots in Roscommon. Note there are multiple registration districts for the county: Strokestown, Athlone, Ballinslow, Boyle, Carrick-on-Shannon & Castlereagh as well as Roscommon itself.
There are limited church records on that same site – none for Roscommon – but still 87 records to look through.
Rootsireland.ie – requires a subscription but has the best collection of parish records for the country. It does not give full coverage and you should also compare with the RC parish records on Ancestry/FMP (which are the same dataset and overlap with Rootsireland)
Failteromhat.com is a wonderful site with a mix of different records but it does have some Roscommon ones.