Hi Gerry,
I know this post is many years old, but i believe we both share an ancestor (John Welsh is my second-great grandfather, my great-grandfather was Hugh Welsh). I would love to hear from you if you are still active. I also believe you maybe incorrect, that it was not Co. Roscommon, but Kilronan, Co. Fermanagh. It is a theory as AncestryDNA states that region under my DNA and the "Welsh" variant of walsh is common there but not common outside of Fermangh and Sligo.
Best Regards,
Rene