a. It’s I.
b. It’s me.
“the choice between the cases for a predicative complement noun phrase varies according to the style level: the nominative is noticeably formal, the accusative is more or less neutral and always used in informal contexts.” (CGEL, p.9)
. . . . . . As for the post-verbal NP in the existential construction, pronouns with a nominative-accusative contrast are rare, but where they occur they are accusative. In answer to the question Who is there who could help her?, one could respond with Well, there’s always me. Nominative I does not occur, indicating that the post-verbal NP has lost the subject case property. (CGEL, p.241)
Does the account say it, there’s always I, ungrammatical? Or while we can use this; when they say there’s always me they denote that the NP, me, has lost subject-case property?
In who is there who could help her?, the former who is shifted forward from complement place (semantic subject; there is who/ who could help her), the latter who is nominative-relative pronoun. In this syntax, which has the accusative case?