"There is water in a mug."
Translation:Tha uisge ann am muga.
The same reason why it’s impractical and immortal and never inpractical or inmortal in English, even though the prefix originally was in-. There’s no grammatical reason for that, rather a phonological one – the last sound of an gets assimilated to the beginning sound of the next word.
It is m before labial sounds: m, b, p, and f (which in Gaelic historically was a bilabial [ɸ] with both lips obstructing the flow of air, but today often is more similar to English labiodental [f] with the lower lip and upper teeth obstructing the airflow).
Yes, those are two English ways to convey the same meaning (but there is water in a mug is more common). To most other languages both will translate as the same sentence since there is no difference in meaning between them (or maybe: because no difference in meaning is between them). ;-)
No word represents there here as it is not needed. It’s just weird English syntax that requires you to put dummy there in such sentences if the subject is indefinite. That is, to say there is water in a mug instead of simpler – but sounding unnatural in English – water is in a mug.
There is no such complication in Gaelic and no need for additional dummy there word (or maybe: no such complication is in Gaelic ;-)).