An portán is "the crab".
The an before the verb (an siúlann) is the question particle (interrogative verbal particle), the an before the noun (an portán) is the definite article "the".
Siúlann an portán - "The crab walks"
An siúlann an portán? - "Does the crab walk? "
In most of the English-speaking world, "Ye" is considered archaic, and you'd be unlikely to encounter it in everyday speech.
Apparently "ye" is still current in parts of England, Ireland, and eastern Canada (Newfoundland and Labrador). It may also occasionally be encountered in certain ethnic/religious communities (e.g., Amish, Quakers) in North America, amongst whom it has been retained as a formality.
Most of the English-speaking world seems to get by without a plural "you", but it can certainly be a useful construction... Other modern versions include "y'all" in the American South and "youse" or "you 'uns" (very informal/non-standard) in the northeastern U.S. And, come to think of it, "you lot" in the U.K.
"ye" isn't encountered much in written English, but it is common in spoken English in some places, and for the purposes of a course on Irish, one of the places where "ye" is widespread in spoken English is Ireland.
"y'all" is getting the "vous" treatment - it is increasingly used as a polite (but informal) singular "you" for addressing someone that you don't know. As such, it isn't necessarily a good translation of sibh and it's associated prepositional pronouns.
Both Irish and English differentiate between the simple present siúlann an portán chugat/"the crab walks to(wards) you" and the present progressive Tá an portán ag siúl chugat/"the crab is walking to(wards) you".
"Is the crab walking to you?" - an bhfuil an portán ag siúl chugat?
Irish and English both differentiate between the simple present and the present progressive. They aren't interchangeable, though not all languages make the same distinction.
An siúlann an portán chugaibh? - "Does the crab walk to you?"
An bhfuil an portán ag siúil chugaibh? - "Is the crab walking to you?"