Irish town house outside school erected (5)
I believe the answer is:
omagh
'irish town' is the definition.
(town in County Tyrone)
'house outside school erected' is the wordplay.
'house' becomes 'ho' (abbreviation for house).
'outside' is an insertion indicator (some letters go outside others).
'school' becomes 'gam' (both can refer to a group of aquatic mammals).
'erected' says the letters should be written in reverse (in a down clue, letters go up).
'ho' placed around 'gam' is 'hgamo'.
'hgamo' backwards is 'OMAGH'.
(Other definitions for omagh that I've seen before include "Ulster town" , "Northern Irish town" , "Town in Northern Ireland" , "Market town in Co. Tyrone" , "somewhere in Northern Ireland" .)