Editing Openai/69186d20-2d14-8013-b9c6-c09000f173f6 (section)

=== Assistant: 2511141715ET ===
251114_1715ET

yes, there are such pairs, but they’re not super numerous, and you have to allow both inherited words and shared Latin loans.

What you want structurally is:

: 

Here are the clearest examples.

==== Proto-Celtic: *koslos “hazel” ====
* Old Irish / Modern Irish: coll “hazel(-tree)” - OIr coll, modern coll /kɔl̪ˠ/, spelled ll (fortis lateral)
* Welsh: collen, cyll “hazel (tree), hazels” - Middle Welsh coll(en), Modern Welsh cyll /kɨ̞ɬ/ with ll = [ɬ]

So here you exactly have:
* Goidelic: fortis ll preserved in spelling as coll
* Welsh: same ll line gives cyll / collen with voiceless lateral [ɬ]

This is pretty much the poster child for what you’re asking.

==== Proto-Celtic: *tullos “hollow, hole” ====
* Old/Modern Irish: toll “hole, hollow” (modern spelling usually single l, but proto-form clearly had ll)
* Welsh: twll /tʊɬ/ “hole” with ll = [ɬ]

Strictly, modern Irish has only one l, so it doesn’t tick your “double letter today” box, but phylogenetically the pattern is:

: 

So this is the same underlying story; orthography just stopped marking the geminate on the Goidelic side.

==== Latin cella “cell, small room” → both branches. ====
* Old Irish: cell “church, monastic cell”; Modern Irish cill /cɪlʲ/ “church (in placenames), graveyard” with ll.
* Welsh: cell /kɛɬ/ “cell” with ll = [ɬ].

So here:
* Source: Latin cella with ll
* Goidelic: cill → /cɪlʲ/, double ll but voiced slender [lʲ]
* Welsh: cell → /kɛɬ/, ll [ɬ]

Not inherited Proto-Celtic, but it’s a clean “double-l in Goidelic vs Welsh voiceless ll” pair, perfect for the phylogenetic intuition: the two branches did different things to a geminate lateral.

==== Putting it together: ====
* Proto-Celtic develops true geminate sonorants (ll, nn, rr, mm).
* Goidelic (Old Irish): - Geminates lose pure length, but are reanalysed as fortis sonorants (tenser, stronger); written ll, nn, rr vs lenis l, n, r. - In modern Irish, the fortis/lenis contrast mostly collapses, but orthographic doubling (coll, cill, etc.) survives as a fossil.
* Brythonic (Welsh): - Some of those fortis lateral / rhotic lines continue pushing rightwards into devoicing: - lateral → ll [ɬ] - rhotic → rh [r̥] - Welsh keeps ll, rh as its own voiceless sonorant phonemes.

So in exactly the sense you care about:

: 

coll ~ cyll/collen and cill ~ cell are the cleanest modern visible pairs; toll ~ twll shows the same Proto-Celtic tull- path once you look one level deeper, even though Irish no longer writes the double.