"Did you walk to school this morning?"
Translation:Ar shiúil tú ar scoil ar maidin?
If you want to be specific about referring to "this morning", the Irish is ar maidin inniu - ar maidin seo is considered béarlachas.
But unless context or tense implies otherwise, ar maidin is read as "this morning". If you are clearly talking about a day in the past, or using the habitual present or past tenses, "in the morning(s)" makes more sense.