Cunctator’s MPSIMS thread got me thinking. When the late Queen Elizabeth, the Queen Mother turned 100 did she get the customary birthday greeting from Queen Elizabeth II, her daughter? After all the monarch is her daughter; one assumes Her Majesty would simply tell Her Majesty “Happy Birthday (Mum)” in person. Do the greetings have to be requested or does the Palace just data mine BDM records and automatically send them out?
One can’t know whether HM the Queen Mother got a telegram, but everyone else gets a card. HM The Queen in the UK and congratulatory messages.
In NZ a request has to be made through the DIA for both birthday cards and wedding anniversaries.
Willard Scott probably sent his regards. 
One assumes that it’s not usually the centenarian herself who makes the request, but a close relative. Like, say, her daughter.
The next question is whether in 15 years she will send a card to herself?
I would guess: no, only British subjects get cards. Just like the monarch doesn’t have a passport nor sing the national anthem.
Well, if she’s abdicated by then, she’ll get a card from the king. If she hasn’t, she’ll probably get cards from several kings and queens.
- Harold van Santvoord, The New York Times, January 8, 1909, slightly modified for this post.
How does the tradition work in the case of a citizen of Northern Ireland, I’m sure a Unionist would be delighted but someone from a Nationalist background might not be as amused.
Since the cards need to be requested, presumably none of the Nationalist’s relatives would make such a request.
(Unless they didn’t like him and did it to piss him off.)
Birthday greetings can be requested by the person or other family members. If you would be offended by receiving such a message, then you don’t ask for it.
This site suggests that the process in the UK is automatic for anyone of the appropriate age who is receiving a state pension benefit. Yet even in these cases the birthday message will not be sent if the person objects to receiving it.