"I read the historical book."
Translation:Léigh mé an leabhar stairiúil.
I can't discern a difference myself between the two concepts in Irish:
"stairiúil, a2. Historic(al); storied. Cuntas <sub>, historical account. Áit </sub>, historic place" https://www.teanglann.ie/en/fgb/stairiúil
"The Foclóir Stairiúil na Nua-Ghaeilge is a project that was started in 1976 with the aim of creating a historical dictionary for Modern Irish." https://en.m.wikipedia.org/wiki/Foclóir_Stairiúil_na_Nua-Ghaeilge
"CLARE COUNTY Council’s visitor information boards in Ennistymon, erected with the support of Fáilte Ireland, proclaim it as 'Baile Stairiúil/Historic Town'...." https://www.irishtimes.com/news/ennistymon-s-historical-blake-s-corner-faces-battle-to-avoid-demolition-1.441507?mode=amp
"ORIENTATION MAP (A4) Killarney. CILL ÁIRNE. BAILE STAIRIÚIL. Historic Town ... Slí Siúil Stairiúil Tras 1 Historical Walking Trail Route" https://failtecdn.azureedge.net/tcs/media/47736594-77ce-4922-b204-d166adf7563a_85656.pdf&ved=2ahUKEwiYjvyy2pnjAhXvRRUIHadBAus4ChAWMA56BAgGEAE&usg=AOvVaw13MpSRKHEwi4OGcq0CqtDA
But I suppose that if you intend to convey that the event is important (i.e. historic), then sometimes to be clear you might just have to specify in Irish that this is/was an important event?