Sorting Whelans from Phelans

Mary Whelan (1832-1901), born Aghaboe, Laois, and died in Pennsylvania. She married John Pound (1827-1877) in the same parish in Apr 1855. Their first son John was born in Dec 1855 in Lisduff.

But Mary Whelan isn't Mary Whelan. As written in Mary's birth record, in her marriage record, and on son John's baptismal record, her name is given as Mary Phelan. Yet for the next six baptisms of her children, all born in Ireland, the mother is cited as Mary Whelan. These six children were born after 1855 and before 1866. No death certificate. Her maiden name of Whelan exists on no known document in the U.S., except the Whelan name survived in some family records.

I notice that Whelan and Phelan are both common names in Laois and adjacent counties. But were the names interchangeable? No. Were there pronunciations in local Laios at the time nearly indistinguishable? Is there a clue in the Gaelic origins of each name?

I've examined dozens of early church records from Camross parish. The handwriting can be misleading in some instances, but the Camross records are filled with the names Phelan and Whelan written clearly enough to transcribe confidently. Sometimes on the same page in the same hand we'll find a couple Phelans and a couple Whelans.

Is there any explanation why Mary switched from Phelan to Whelan? Was it a deliberate move? Since Mary was illiterate (per 1860 U.S. census) she may never have noticed the variation in how her name was written.