I'd say the starting point for your search should be probably be to presume the surname is Mc Lean. Note that the informant is not a relative on either of those d/cs.
As you have three certificates which give slighty different names, if I were you I would seek out a marriage or birth certifcate, probably Henry's, to double check Rachel's surname.
When searching for McLeans also consider variations in spellings such as Maclean etc. irish and Scottish spellings can vary a lot. Rachel is often spelled with an 'a' so maybe use an asterisk when searching for Rachel too.

If Rachel has been in NZ for 40 years you can estimate year of arrival to look for her immigration if you haven't already done so. Added: I see that Okrad has located this for you.
After you determine Rachel's maiden surname, start a thread on the appropriate Irish board for tips for further regarding the families. Provide a link to any related threads on the families so others know what information you have already been given. Don't get your hopes up though, as Irish family can often mean a dead end.