There is a townland named Mayo in County Leitrim, and you can find more info about it at this site:
https://www.townlands.ie/leitrim/carrigallen/oughteragh/ballinamore/may…
A townland is a smaller area within a parish, and the townland of Mayo is in the parish of Oughteragh. The parish records for Oughteragh only go back to 1869, unfortunately, and can be found at this site:
https://registers.nli.ie/parishes/0816
It's interesting that your ancestor ended up in a townland named Mayo (in irish, Maigh Eo, meaning "yew plain"). In Irish, the surname McAndrew is Mac Aindriu. It was adopted by a branch of the Barrett family in County Mayo, which is separated from County Leitrim by County Sligo and County Roscommon. The Barrett’s were an Anglo-Norman family which became fully Hibernicized (the name was sometimes re-Anglicized as FitzAndrew). They became like a native Irish sept (or "sub-tribe"), having a well-defined territory on the eastern side of Lough Conn in northeastern Mayo (in the area of the current parishes of Backs and Attymass), and they intermarried with the other families in the area for centuries (I have a McAndrew great-grandmother from Mayo myself). In Griffiths Valuation, in the early mid-1800's, there were 232 McAndrew tenants or landholders listed in all of Ireland, of whom 186 were in County Mayo, and 19 in adjoining parts of County Sligo, with a few also in Roscommon. Perhaps a few also migrated to Leitrim (which is not all that far away), but all all 16 of the McAndrew births registered in 1890 in Ireland were in Mayo, so your Mary McAndrew may have been born in one of those neiighboring counties.