The current situation

Haplogroup L-161 is a tiny group that comprises less than 1% of the European population. It is found predominately - more than 95% - in the British Isles. For that reason, the label "Isles" has become attached.

The branches of Haplogroup I that have led to the modern L161 subclades are:

 ⤷—➤ M438—I2
——— ⤷—➤ P37—I2a1a
————— ⤷—➤ M423—I2a1a2
——————— [⤷—➤ Y3104 —I2a1b2 removed by ISOGG 2018, retained by Yfull ]
———————⤷—➤ L161—I2a1a2a
—————————⤷—➤ S2639—I2a1a2a1 —Isles AB
—————————————↓  ⤷—————➤ Y12072 I2a1a2a1b ➤ PF4135 - Isles A I2a1a2a1b1
————————————— ⤷———————➤L1498 - Isles B I2a1a2a1a
——————————————— ⤷—➤ Y3734 - I2a1b1a1a1b3
—————————————————↓  ⤷—➤Y5450 - Isles C - I2a1b1a1a1b3a
————————————————— ⤷———➤  Y4665 - Isles D - I2a1b1a1a1b3b

Isles A is concentrated around Cork and is present, but rare in Ireland, Isle of Man, and Scotland.
Isles B is now found all across the British isles and to a very minor degree on the continent.
Isles C is located mostly in the north of England, Scotland, Ulster and western Ireland.
Isles D is relatively strong in the north or western parts of Ireland, especially around Rathcroghan.

The I Group as a whole has a very different current distribution. Most are in Europe sandwiched between Haplogroup R1a on the east and R1b on the west. It is not at all clear how that came to be. There are many diverging opinions

Predominant European Haplogroups