what precipitated the loss of dental fricatives in 'middle irish' around 700 years ago?

Were there 4 or 2 forms?

You mean the th-sounds? I never even knew Irish used to have them. (Modern Welsh has them, though.)

there were due to dental /t/ and /d/ (sorry no gentium on this station nor will it allow their installation, so imagine the dental diacratics) shifting under lenition Wikipedia gives 4 (http://en.wikipedia.org/wiki/Old_Irish).

I gues this meant there were either 4 sounds articulated on the teeth /t/ /d/ and /t'/ and /d'/ (the last two seem improbable), or a shifting from fricative /d'/ and /t'/ to /θʲ/ and /ðʲ/ as they are on wiki under lenis contexts.

There also were gemminated consonants too.

The interesting thing about Gaeilge Theilinn by Wagner is that he records dental fricatives under sandhi in older speakers in the 1940s, suggesting, i suppose like nasalised bilabials vs. non-nasalised bilabials in older gaeltacht speakers today, that after a range is no longer phonetic, it can continue on incidentally transmitted by purly mechanical means over the generations.

He mentions gemminated consonants too in Donegal of that period...

> what precipitated the loss of dental fricatives
> in 'middle irish' around 700 years ago?

No idea. Sound changes happen, whyever.
German lost [γ], [θ], [ð]. Welsh lost [γ]. English lost [x] and [γ].
Etc.

> Were there 4 or 2 forms?

Probably 4 ([θ], [ð], [θ´] and [ð´]), because they've been replaced by 4 (or at least 3) different sounds (i.e. usually [h], [γ], perhaps [h´] and [γ´]).