To refer to our mothers, we lovingly say: "Siya an ako nanay" (She is my mother). I would be offended if a friend suddenly asks me, "Hain an im iroy?" (Where is your mother?). Nanay and iroy are synonymous, but we use them in different ways and in different occasions. In ordinary conversation, we refer to our mothers as nanay, not iroy. The latter suggests disrespect for the person (mother).
It’s interesting to note that we use the same word (iroy) when we want to be dramatic or when we are emphatic, particularly when delivering a siday or lowa (poetry). Kanta Para Sa Akon Iroy ( ASong for my Mother) and Mga Rayandayan San Akon Iroy (My Mother's Wealth) are acceptable because they can pass as titles for a literary work. The Philippines is not our nanay nga tuna. We always refer to it as iroy nga tuna (motherland) as in “An iroy nga tuna matam-is pagpuy-an.” ("How sweet it is to live in our motherland!). The same word is used in reference to Mary as in, "Santa Maria, Iroy sa Diyos" ("Holy Mary, Mother of God") and in, "Maghimaya ka Rayna, Iroy san Kaluoy" ("Hail Holy Queen, Mother of Mercy").
Nanay is conversational and informal; hence, we use it in everyday language and in ordinary discourse. Iroy, on the other hand, is formal. While it is true that “iroy” is a swear word, it is also the literary term for mother.
Would you like to know how to pronounce the words nanay and iroy correctly? I made a video for you. I also provided subtitles.