Hi Judy,
A check on www.rootsireland.ie did not show any Johannah McMahon whose father was named William - 1820 - 1840.
I830 - 1840 there are 22 Johanna McMahon - 15 in Limerick and 3 in Co. Clare. The others in Tipperary, Cork, Kerry. The 3 in County Clare are in Killadysart. There is a Maria McMahon in Cratloe in1832 whose father is William and mother named Maria Hayes. There is a John recorded in 1837. Cratloe is close to Limerick city.
No record found for the marriage of John McGrath and Johannah McMahon. There is a record of the marriage of a John McGrath to a Margaret McMahon in St. Mary's R.C. Church, Limerick on May 15th 1858. Parent's names not recorded.
If you find any other information in relation to the couple, you can come back.
Regards,