Except when it's used with nach or an or más or ba.
Examples from focloir.ie:
"that's a bit of a problem" - fadhb bheag is ea é sin
'Is that a problem?' 'I guess not.' 'An fadhb é sin?' 'Is dócha nach ea'
"Boastful? Moi?" - Gaisciúil? Mise, an ea?
"go on then, since you're that clever!" - seo leat más ea, ós rud é go bhfuil tú chomh cliste sin!
"it was a momentous day" - lá mór ba ea é
They are not interchangeable.
You use tá if you are agreeing with or affirming a sentence that used the verb bí, you use is ea when agreeing with or affirming a sentence that used the copula.
(Though strictly speaking you should only use tá, without the sé. Tá sé without anything following it doesn't normally occur).