I was in Ireland last month near the northern part of the Republic of Ireland. 100% of all properties were bordered by either a nice hedge or a wall of some form or another. Even the big fields had a thicket surrounding each one. Is there a land surveying type of law that establishes this or is the reason for this purely non-legal?
Good fences make good neighbors :) https://www.poets.org/poetsorg/poem/mending-wall
But seriously, hedges were planted to form property boundaries between counties, church lands, private lands, communal grazing lands, etc. A tall, thick hedge obviously and clearly marks a boundary, stops grazing animals from straying, and provides privacy between farms.
Hedges were fairly cheap, could not be carted off like a lumber fence, and required little maintenance. I'm also sure you saw freestone rock walls; same property line function, different effort and effect.
There may have been laws that required property boundaries to be marked by hedges in years past. (Someone with more knowledge of UK laws can respond). But it may simply have been form follows function.
There are current laws that protect hedges from being cut down without good justification, as well as laws that only permit trimming of hedges during certain times of the year to protect the wildlife that inhabit the hedgerows.
The farm land hedging could be explained by the GLAS scheme where you get paid €5 for every meter planted for 5 years