In the Standard, you have:

ar an siúcra (masculine)
ar an tsráid (feminine)

I've heard though that the majority of speakers say:

ar an tsiúcra (masculine)
ar an tsráid (feminine)

What percentage of people don't change it to "tsiúcra"? Is it a minorty like 40%, or is it something like 5%?

As a person learning Irish, would you be better off to ignore the Standard on this one and mutate for both masculine and feminine?

quote:

ar an tsiúcra (masculine)
ar an tsráid (feminine)

Such pronunciation is found in Tír Chonaill only.

In Cois Fhairraige you have the same as in standard.

In north Connacht (+Mayo) and Munster you have:

ar an siúicre
ar an sráid.

Except for standard with its idiotic rule, the rule everywhere is straightforward - "s" becomes "t" after article when lenited.

That is why you have "ts" in Tír Chonaill, as they always lenite after prep+article, and you don't have anything in Munster - as there is urú here, so no "ts" is possible.

Alternatively after sa:

sa tsaol in both Munster and Tír Chonaill.
sa tsráid

the same rule goes for other similar situations (lenition of "s" after n" -

aon tsaghas, aon tsaol