Is deas liom bualadh leat. Tá sé deas bualadh leat.

I've seen these both used in the same Rosetta Stone conversation. Why would one use Tá and the other use Is?

Well, I would be more likely to say the first version but the second is not wrong either. You could drop the "liom" from the first one.

There are instances where the copula (is) and substantive verb (bí) are interchangeable. Tá an lá go breá - Is breá an lá é, though the second is more emphatic. Note how the adjective is brought to the front to emphasise the loveliness of the day.

Sometimes the copula and substantive verb mean slightly different things: "Fear mór láidir is ea é" (his general state) but "Tá sé ina fhear mór láidir ó chonac cheana é" = He has grown up into a big strong man since I last saw him (a change in state has occurred).

"It's nice for me to meet you."
"It's nice to meet you."

People everywhere just like saying the same thing different ways.

I have noticed "is deas bualadh leat" more and more. But when I first started learning, in 96, I learned "Tá áthas orm bualadh leat". Has that become too formal? Or are people just more keen on saying "Is deas bualadh leat" because it sounds more like english's "its nice to meet you?

quote:

I have noticed "is deas bualadh leat" more and more. But when I first started learning, in 96, I learned "Tá áthas orm bualadh leat". Has that become too formal? Or are people just more keen on saying "Is deas bualadh leat" because it sounds more like english's "its nice to meet you?

No. The construction "Is deas le" is used. All these constructions with Is + adjective/noun + le + personal pronoun all mean "Is considered/felt by me to be", so "Is maith liom é", literally means "It is considered to be good by me" i.e. "I like it". "Is breá liom do chasóg" - "It is considered to be lovely by me - your coat", i.e. "I love your coat". This is + le construction expresses a personal feeling or impression or preference. Consider the following two expressions:

Is mór agam é = I value it, appreciate its importance/worth
Is mór liom é = I think it's rather big
Ní mór liom duit é = I don't begrudge you it (but I'm probably giving you too much information at this stage)

A common mistake for learners is to say "Is grá liom do chasóg" etc. This literally means "Is considered a love by me your coat" which makes no sense. The correct form is "Is breá liom do chasóg". Another form is "Is geal liom é" - it warms my heart.

Is geal leis a bhfeiceann sé; he likes (everything) he sees; he sees things through rose-coloured glasses! (Ó Dónaill)

Is geal leis an bhfiach dubh a ghearrcach féin; literally "Is considered bright/joyful by the raven its own chick" i.e. Beauty is in the eye of the beholder!

"(but I'm probably giving you too much information at this stage)"

I actualy like all the info I can get.

I do remember the Ní mór liom duit é and Is mór liom é from Dillons TYI.

they also give

Is áil liom = I wish
and in Ó Dónaill - Mar is áil leat = as you wish.