Remind me of how in modern Irish, a prefixed t- appears before vowels in masculine nominative singular nouns. It relates to Proto-Celtic masculine nominative s

Remind me of how in modern Irish, a prefixed t- appears before vowels in masculine nominative singular nouns. It relates to Proto-Celtic masculine nominative singular article *sindos. The t- is what remains of the d and s from Proto-Celtic: an t-athair - the father an t-úll - the apple an t-each - the horse an t-ospidéal - the hospital