The prefix "irgend-" means "any". It normally is combined with question words, like "irgendwo" (anywhere), "irgendwann" (at any time; "wann" = when), irgendwer (anyone; "wer" = who) or, as in your second example, "irgendwelche" ("welche" = which). However, for whatever reason, the latter is only used in the plural form; in the singular form instead of the singular "welcher"/"welche"/"welches" an indefinite article ("ein"/"eine") is used, so you get "irgendein"/"irgendeine". Since the article is singular, of course it's used only if the following word is singular. The plural of "irgenein" is "irgendwelche".
Note that the meaning changes in the obvious way whether you use the singular or plural form (although in your example, it's not obvious: Whoever says "wenn Sie irgendeine Frage haben", will probably not complain if you have actually more than one question). For example
Nehmen Sie sich irgendein Buch
means you should take exactly one book (but it doesn't matter which), while
Nehmen Sie sich irgendwelche Bücher
means you may take several books.