Céard é seo?
Cad é seo?

Which of the above is correct to use?

Also Seo í/ Seo é depends on whether the object is masculine or feminine. Is this correct?
Thanks

Both are equally correct in speech, but I'm not sure about writing. Cad é seo? is certainly more common, but that doesn't mean that Céard é seo? is actually incorrect. Céard is listed in de Bhaldraithe, which is my usual litmus test for determining whether something is CO or not.

In English you have two ways of saying "cén fáth"; you can say "why" or you can say "how come". Not only that, but it's not dialect-specific, i.e. you can hear a Dublin speaker say "why" in one sentence and then "how come" in another sentence.

Same situation here, you'll hear someone say "cad é" one minute and "céard é" the next... there could possibly be an underlying reason why they tend to choose one over the other, but if you need to analyse something that closely then I think it's best to "get a feel for it" rather than trying to think about it.

There's a lot of things in language which you just need to "get a feel for"; using emphatic pronouns such as "mise" is a good example.

Cuigeadh Mumhan :- Cad e seo ?

Cuigeadh Chonnacht :- Ceard e seo ?

Ce rud e ? > Cer'd e ? > Ceard e ?

A Dharaigh, ná fuil síneaidh fada ar aghaidh boise agat?

Giorrúchán "co tá" (= cad é tá) is ea "cad".

An bhfuil difriocht ann idir "Ca" agus "Cén áit"?

Is beagán aisteach dom ná fuairis freagra ar an gceist seo fós. Táim den tuairim--agus is féidir ná fuil an ceart agam, ar ndóigh--gur "cá" an focal is coitianta i nGaelainn na Mumhan agus gur "cén áit" an focal is coitianta i nGaelainn Chonnachta. (Níl a fhios agam cad é is coitianta i nGaelainn Uladh).