Translation:I am stealing IRN BRU and a bonnet.
It is not needed for Irn Bru. The definite article "an" is used in the following circumstances:
before feminine words beginning with a vowel.
before masculine nouns beginning with the three consonants c, g and s - before masculine and feminine nouns
beginning with the consonants d, l, n, r and before feminine nouns beginning with the letter s apart from those beginning with sl, sn, sr, or s+ vowel (see below for information on these nouns.)
before feminine nouns beginning with the letter f followed by a vowel. A word like this requires the letter h to be inserted before the vowel when using the definite article. Take faoileag (a seagull), for example. It becomes an fhaoileag.