About this phrase

Tá is the present tense of the verb bí, meaning be.

Orm is the prepositional pronoun, first person singular, formed from the preposition ar, meaning on.

Tá orm carries the actual meaning I must. It's essentially meaningless literal translation is is on me.

Prepositional pronouns formed from the preposition ar

• orm: on me.
• ort: on you.
• air: on him; on it (masculine).
• uirthi: on her; on it (feminine).
• orainn: on us.
• oraibh: on ye (plural).
• orthu: on them.

The same prepositional pronouns' emphatic forms

• orm-sa: on me.
• ort-sa: on you.
• air-sean: on him; on it (masculine).
• uirthi-se: on her; on it (feminine).
• orainn-ne: on us.
• oraibh-se: on ye (plural).
• orthu-san: on them.