Yes, the general rule is that proper nouns that end in < y > simply add < s > to form the plural; the base remains the same and the word takes the regular suffix. Hail Mary is a proper noun, so it follows the rule. This rule applies to common nouns that are used as proper nouns, e.g., three copies of Variety magazine would be referred to as three Varietys.
Common nouns typically drop the < y > and add < ies >. This applies to common nouns whose etymology is a proper noun, as in johnnies (condoms) or jimmies (ice cream sprinkles). Exceptions are common nouns with composite vowels, which simply add < s >, e.g., guys, honeys, donkeys.
As in all things English-language related, there are exceptions. One is money, which typically does not inflect the plural at all, but when referring to multiple types or sources of funds is spelled monies, not moneys, an exception to the exception.