Translation:Uisce, le do thoil.
Most Irish consonants come in two varieties: “broad” (velarized) and “slender” (palatalized). A rough analogy in US English would be to compare two pronunciations of “coupon”; some people pronounce it as “coo-pahn” (using a “broad” C), and some people pronounce it as “queue-pahn” (using a “slender” C). A better analogy would be the distinction between hard consonants and soft consonants in Russian.
Whether a given Irish consonant is pronounced slender or broad depends upon the nearest vowel to it. If the nearest vowel is e, é, i, or í, then it’s a slender vowel, and the consonant is pronounced slender; otherwise, the nearest vowel is a broad vowel, and the consonant is pronounced broad. Note that the digraph ae is considered a single broad vowel for this rule. Also note that there are occasional exceptions to this rule; the most common exception is that the nearest vowel excludes those that cross a component word boundary in a compound word. For example, in the word sobhéasach (“well-bred”), the bh neighbors both the o (a broad vowel) and the é (a slender vowel). But since sobhéasach is a compound word, made from so + béasach, the so is ignored in determining the nearest vowel to the bh; thus, the é is the nearest vowel to the bh, and the bh is pronounced slender.
It's never "hoil".
It's le do thoil if you are saying "please" to one person, le bhur dtoil if you are saying "please" to more than one person, because the singular possessive adjective do lenites the following word, so toil becomes thoil, whereas the plural possessive adjective bhur eclipses the following word, so toil becomes dtoil,