Trying to instantiate a DS3231 real time clock within a library
You appear to be providing a pointer to a constructor, not to an object.  Either create a new object with new, or instantiate the object separately and provide the pointer to the object:

_realTimeClock = new DS3231(SDA, SCL); 


Better still, extend the existing class and implement your own methods:

clsss JBP_DS3231 : public DS3231 {
    public:
        JBP_DS3231(uint8_t d, uint8_t c) : DS3231(d, c) {
            String thisTime = "";
            Serial.println("About to try to get time from RTC."); // Last line we hear from.

            thisTime = getTimeStr();
            Serial.print("Time in constructor is: ");
            Serial.println(thisTime);
        }

        String TimeStamp() {
            String timeStamp = "";

            Serial.println("In TimeStamp().");
            timeStamp = getDateStr(FORMAT_LONG, FORMAT_MIDDLEENDIAN, '/');
            Serial.print("Value of timestamp is: ");
            Serial.println(timeStamp);
            timeStamp.concat(" ");
            timeStamp.concat(getTimeStr());
            Serial.println(timeStamp);

            return timeStamp;
        } 
};

void setup() {
    Serial.begin(115200);
    JBP_DS3231 myRTC(SDA, SCL);
    Serial.println(johnsRTC.TimeStamp());
}

void loop() {
}


(Note: untested)