I've noticed that the th sound often becomes a plosive sound in Appalachian English. When and how did this phenomenon start?The only case I know where this happens in the british isles is Irish.Does it also happen in other dialects on the island of Great Britain?
Th-stopping has always been a distinctive feature of Irish English, where the phonological distinction between [t] and [t̪], [d] and [d̪] is mostly maintained. It is not characteristic of most varieties of English in either England or Wales; urban and working-class English speech in southern England prefers th-fronting instead.
However, young urban English in England, not limited to modern Multicultural London English (MLE), is tending more towards a complete merger of /θ/ into [t] and /ð/ into [d], conventionally called th-alveolarisation but often subsumed under the above. The effect is stronger with the definite article
the and determiners
that, and is usually attributed to Caribbean and AAVE influence on MLE and the speech of other English urban areas, and is becoming more common among speakers of different ethnicities.
Rural Irish accents also have this complete merger, and this may have been the source of th-stopping/alveolarisation for many English phonologies across the North American continent, notably in Newfoundland English, where it is attested from the late 18th-century.
In Scotland, th-stopping is mainly found in Glasgow and rural communities (e.g. the Shetlands), and is attributed to Irish/Ulster influence; th-fronting is more common; th-debuccalisation (where /θ/ and /ð/ become [h]) is distinctive to Scotland. This is most probably the source of th-stopping/alveolarisation in Appalachian English: Ulster Scots, although there are relatively few historical records of the speech.
However, it is to be mentioned that th-stopping has been a tendency for a long time - at least since the first few centuries CE in Germanic (the Second High German Consonant Shift): across Dutch, Low and High German and continental Scandinavian, but not Icelandic or English (Faroese lost it subsequently and separately). Compare German
der with English
Dorn with English
Tod with English
Even in Early Modern English of the early 17th century, the tendency to merge into /t/ and /d/ was ever present, such that certain mergers were accepted into the standard, e.g. English
murder vs Icelandic
burden vs Icelandic
byrði. Shakespeare himself used the "th" variant in Othello in 1622 (Act I, Scene 2):
Yet do I hold it very stufe of Conscience
To doe no contriu'd murther
... but in the 1623 printing, it is uses murder.
By the 18th century, the th-fronted form has won out in London at least. From The Beggar's Opera (1728):
You never had a finer, braver set of Men than at present. We have not had a Murder among them all, these seven months.
The question why is always tricky in diachronic studies, and I think articulatory phonetics does play a role, but it's often hard to pin down what "effort" means. There is also the auditory/perceptual phonetics - somewhat less well-studied. Language contact is also a major factor, and cross lingustically /θ/ and /ð/ are pretty rare (and not really found in modern Standard Irish nor in Ulster Gaelic - it was lost after Old Irish via debuccalisation, but the modern orthography of Irish still makes reference to it).