When writing, it's always important for each character to have their own distinct personality. One easy way - though often overlooked method - is through their speech patterns.
Black Swamp Mysteries series has such a voice; she is soft-spoken, which shows her gentle and loving nature.
In contrast, Vicki's sister Brenda Carnegie is often sardonic, her words clipped but silky, showing her street-savvy side but also her passionate side. She is more likely to speak in sexual innuendos and double entendres.
Dylan Maguire's speech patterns are unique because they show an Irish heritage though he lives in America. His speech is that of rural Ireland, in which g's are more likely to be dropped, and "me" or "m' " are used instead of the word "my".
Ryan O'Clery, the main character in The Tempest Murders, is also an Irishman but he is a Dubliner. His speech patterns are not those of Dylan's at all, but those of urban Ireland, of education and intellect.
When writing a character's speech in a pattern that is easily identifiable, it is vastly important to select one or two characteristics to stick with, instead of overwhelming the reader with abbreviations that slows them down as they seek to decipher it.
I recently read a book set in 15th century Scotland, in which the author had so many inflections in one character's speech that I felt like I was reading another language. Try deciphering this dialogue: "Nay, an' I dinna ken 'e be kepin' ta ye se'f 'sted o' suppin' w' me."
In case you have no clue, the dialogue actually means: "No, and I don't understand why you'd be keeping to yourself instead of having supper with me."
When a character is difficult to understand, it's best that they be a minor character, in which their speech makes them easily identifiable but they don't have so much dialogue that it slows down the action.