As a 'Mercan zombie in London many moons ago, I was taught by proper English zombies to add milk to the cup first.
Have had milk (well, half & half) and sugar in my tea ever since.
Hot black Indian tea, milk, sugar. Nothing’s better fiest thing in the morning and when the afternoon’s dragging. I’m sipping one right now and it feels soooooo good.
Following up what Eve said, for a terrific pot of tea use filtered water. And get the temperature right: it should be 180 degrees F for most black teas. Some green teas work better with cooler temperatures.
A tiny bit of sugar can take the bitter edge off of a green tea. Oddly enough, you can put in so little that the sweetness cannot be perceived, just the reduction in bitterness.
On the OP:
This is a trans-Atlantic controversy.
American: Tea, sugar, then milk. “The hotter temperature permits the sugar to dissolve faster.”
Brit: Milk, tea, then sugar. (Pause.) “It mixes better.”
MfM: I’m not sure whether I have a preference, but the 2 methods have a perceptible taste difference. (Pls forward hypothesis to Unalabs for evaluation. 
)
I’m a Brit, and I put the sugar in first, then pour the tea. Dissolves best that way.
As I understand it, Irish tea is of a higher quality that UK tea, and teas sold here are of a particular mix of Assam and other teas. Obviously quality is a subjective thing, but Ireland is one of the highest users of tea per head in the world. In the past, it was said to be the highest - but coffee has made substantial inroads into the usage.
Many British brands of tea have launched in ireland, but failed to achieve any market share, because they were perceived as being of poor quality.
Google “Irish tea” for more details. Some are self-serving sites of Irish tea-makers, but you will get the idea.
Whether it is coffee or hot tea, I always add the milk after. Not because I learned that it was the ‘right’ way to do it, but because that’s the best way foe me to see if it’s the proper amount. I know what color the beverage should be in order to have it to my taste, so I just add however much milk makes it look like that.
-milk - sugar - liquid.
- vodka - ice - liquid
saves on spoons
I put the milk in first as a matter of practicality so I know exactly how much I added.
But isn’t the important factor the ratio of milk to strength of tea, not the absolute quantity of milk? And isn’t that best judged (titrated) by the color change as the milk is added?
Irish tea just has a higher proportion of assam leaf - I like that personally as I generally drink pure assam.
English Breakfast tea is a mix of assam, kenyan and ceylon teas. Kenyan tea is probably the weak link here (read cheapest).
I am not sure quality is lower but more the blend is different.
I think the problem with tea outside the British Isles is Liptons - it is poor quality tea IMHO certainly what they bag.
Liptons has never succeeded in making a market in Ireland, although it sells in the UK. That supports your point - and mine.