"An t-ìm blasta."
Translation:The tasty butter.
No, when you think about it, there’s nothing easier in saying an t-ìm than an ìm ;-).
That’s just how the masculine definite article works in Gaelic, it prefixes t- in nominative to words beginning in a vowel: an t-athair the father, an t-ubhal the apple, an t-ìm the butter. But not in genitive: an athar of the father, an ubhail of the apple, an ime of the butter, nor in dative: ris an athair with the father, air an ìm on the butter.
Nor in feminine gender: an oiteag the breeze, an uair the hour.
Historically this t comes from the article itself, in masc. nom. sg. it was *sindos which then lenited s to *(s)indoh, then lost unstressed o: *(s)indh, and then devoiced the d: int, and that’s the Old Irish form before vowels. In Old Irish the horse was int ech (modern an t-each).
Since generally the article lost this final d/t, it got reanalyzed as a part of the following noun (hence modern orthography an t-ìm and not
You can read more about it on Gramadach na Gaeilge – website about Irish grammar, but the history here for Scottish Gaelic is the same.