"an t-slige ghorm"
Translation:the blue shell
It’s an exceptional lenition pattern after the article.
Basically in Old Irish the article was actually in, ind, int (from older forms *sinda) and it caused the regular lenition of s, so let’s take the word for eye, in modern Gaelic sùil (fem.), in Old Irish súil, with the article it used to be int ṡúil pronounced /int huːl’/, but the h was weak so it disappeared, the int ṡúil becoming pronounced /intuːl’/.
Before other consonants the d, t disappeared, the article becoming an, and the t that remained before words with s- was re-analyzed as a mutation of the s- itself /ən tuːl’/ and started to be written as such: an t-sùil (or an tsúil in Irish).
That’s also the reason for t- appearing before masculine nouns starting with a vowel (eg. an t-athair).