full speed in the wrong direction

literally just saying 'he spoke with a Welsh accent'; sure, it's a bit blunt, but it gets the job done in a pinch. "He's completely drunk," he said, his southern drawl lingering on the final syllable as if to highlight the extent of the offence. Y'know, something of that ilk, but not as shit.

learning the specific vocabulary and syntax that someone with that accent might use. Sticking with the Welsh theme, because it's objectively the best accent*, there's a bunch of things that differentiate a colloquial South Walean accent, outside of our famed tendency to elongate a vowel to the point of death. The way we use prepositions (where to by is he?), the vocabulary borrowed from Welsh - saying that someone daft is twp, or something small is dwty - can easily signpost our speech as being from that specific area, without needing to type something like "'e's absolutely 'angin', man, pissed as a faaht 'e is!" Something less jarring, such as "He's absolutely hanging, he is." is just as clear. A character who says "Do you want a cuppa?" is coded or located very differently to one who says "You'll have a cup of tea, so you will."

ditto if there are specific ways that someone from a certain area might refer to a well-known concept. Regional words for mother and father, for example, or words that are class-specific; your character who calls his parents 'mater and pater' is likely inhabiting a different socioeconomic strata than your character who calls them 'mam and dad'. See if there's a colloquial way of saying 'yes' and 'no'; a lot can be signposted if your character says 'nah' rather than 'no', or 'aye' rather than 'yes'. A character saying 'couch' is inherently coded differently to one who says 'sofa'.

The reasons that writing accents phonetically is Generally Ill-Advised, In My Opinion are as follows:

quite simply, you're probably not being as clear in conveying the sounds of the accent as you think you are. Taking JK Rowling's work as the best possible example of this, her attempts at writing a Cockney accent phonetically come across like someone is chewing a mouthful of cheese curds and struggling to contain them. There's no consistency, no proper understanding of how to transcribe syllables into writing in a way that coherently conveys the accent she's trying to portray. I mean this so seriously, but what the flying fuck is: 'Well, 'e 'ad these 'ead pains and 'e was def'nitley nervous. Depressed maybe.' It's a crime, is what it is.

it's just plain hard to read. Trying to wade through sentences full of apostrophes and elision, parsing what's actually being said, gets tiresome. It asks the reader to do work that you're actively making harder for them. And that's not always a bad thing! Making readers Put Some Fucking Effort In can be very fruitful! But do you really want them to be struggling to understand every single thing that your Character B is saying for 350 pages?

which leads me onto the last point, and the most important in my mind: writing out accents like this always, always affects accents that are already in some way Othered. They're either racialised or working class, or associated with certain local regions that have negative stereotypes - think the deep South of the US, or the Welsh Valleys. They're never the 'default'. And this raises thorny questions about what the default is, what the standardised accent is, the accents that do and do not merit differentiation from the norm. You're relegating Character B to being hard to read because he's from, idk, Sunderland. You've decided that he isn't speaking 'properly', and therefore the reader needs to understand that other people think he's speaking weirdly. That, to me, is the principle issue. Because returning to JK Rowling (a sentence I hoped never to type), the only characters who speak like this in her work are working class, or they're from other countries. They're never from, you know, Surrey. Wonder why that is. And it's easy to be glib about it, but I do think it reifies class and regional boundaries in a way that's ultimately harmful.

This isn't to say that there's never a place for eye dialect in writing - Trainspotting, for example, wouldn't be what it is without it, and there's definitely a different conversation to be had when it's your own accent and you're making a deliberate point about identity by differentiating through eye dialect - but I think that the blanket assumption of 'oh shit, my character is from Ireland, I'd better type that out phonetically!' can actually be both damaging to your writing and to your character representation, and I think that instead doing the work to really understand the vocabulary, speech patterns and unique aspects of a language or dialect always makes a work feel more authentic and lived-in.

To wit, less of this shite:

There’s mony a slip, an’ I’m no losin’ sight o’ any o’ my suspectit pairsons, juist yet awhile. (Peter Wimsey, if you were wondering, and yes, that's supposed to be Scottish)

and more of this:

"Are we straight so?" "Aye, we're straight," said Jim. "Straight as a rush, so we are." (Jamie O'Neill, Irish, from At Swim, Two Boys)