Pious type, one with seat in Ireland, unified states once (4,5,6)
I believe the answer is:
holy roman empire
'unified states once' is the definition.
(historical collection of states)
'pious type one with seat in ireland' is the wordplay.
'pious' becomes 'holy' (both mean religious).
'type' becomes 'roman' (variety of type eg Times New Roman).
'one with seat' becomes 'MP' (someone with a seat in parliament).
'in' indicates putting letters inside.
'ireland' becomes 'eire' (poetic name for Ireland).
'mp' going within 'eire' is 'empire'.
'holy'+'roman'+'empire'='HOLY ROMAN EMPIRE'
(Other definitions for holy roman empire that I've seen before include "Charlemagne ruled it first" , "unification of Europe" , "earlier power?" .)