There's a well-worn (and it's true which is why) phrase - 'Pick your battles ...'
and this is what Mr McClean needs to understand.
There is nothing wrong with being proud of your roots, country, and culture, but there are times and places to demonstrate that feeling, and this was not either of them.
If you undertake to play football for a British team in international competition, then you will hear, and be expected to acknowledge the National Anthem.
If you feel unable to comply with that situation, then restrict your career to Irish clubs, or request not to be chosen for international matches.
You cannot accept the career, money, respect, and adoration that goes with being a top footballer, and then decide to stand on your pride in a situation like this.
It makes you look mean, stroppy, immature, and incapable of adult judgement - and on this occasion, Mr McClean has demonstrated all of those facets of his character.
Football is a team sport - if you can't be with your team through its various responsibilities - then take up golf or darts, or some other sport where your respect for your fellow players is not required to be demonstrated in these situations.
Death threats are ludicrous, and utterly unacceptable.