Thats a shame - you must determine the specific parish or townland from which your ancestor came - obviously made difficult, primarily due to the destruction of the Record Tower in Dublin Castle in the early 18th century and the disastrous 1922 fire in the Ireland Public Record Office, which nearly obliterated civil records. However, you have a few clues about that which is more than most people seem to have. According to the Griffith Valuation 1847 - 1864 there were'nt that many Morrisroe families in Roscommon and Mayo
We all have different ways of working but for me, if you can't make progress one way then you need to find an alternative, casting a wider net and looking for siblings, cousins, friends, and neighbors. Like most communities at that time families stuck together in groups. The goal is to get down to the specific townland in Ireland, including parishes, baronies, poor law unions, dioceses, probate districts, and counties. May even end up as a One Name Study to accomplish your aims and even then it may still be a "proverbial brick wall".
Wishing you all the best with your continuing research.
Regards
Sandra