so I've seen the following constructions

Gráim thusa (I love you)
Tá mé i ngrá leat (I'm in love with you)
Tá mórán grá agam


but

what would follow the third one?

tá mórán grá agam leatsa(?) duitse(?) ortsa(?)

also, could I say "táim lán grá..."? and the same question of what preposition to use...

Both "duit" and "ort" are acceptable. Haven't heard "leat" before, but that hardly means it's never used.

I don't see why you couldn't use "a lán" there, but that's not my dialect. (I'm comfortable with "mórán", but some varieties only allow it in the negative.)

I was once told by an Irish speaker that an expression such as gráim thu would be considered agressive and somewhat presumptious while tá grá agam duit is more genteel and therefore more appropriate.

I love you (whether you like it or not) vs. I have love for you.

Perhaps that's a cultural issue. Or maybe it's just outmoded Jane Austinism.

It's quite likely cultural. Although 我愛你 wǒ ài nǐ has been gaining ground in Chinese due to contamination from English (often through the pernicious influence of song lyrics), I still know older Chinese-speakers for whom this is simply too bold and who would much prefer an older, more indirect idiom.
Then again, Irish seems to have a penchant for the periphrastic in general. Although there exist simple verbs like cuimhnigh and basaigh, I don't see them used nearly as often as expressions like is cuimhin le/tá cuimhne ag or faigh bás (respectively).

"Tá mé i ngrá leat"

That must be a calque

I'm pretty sure it is, tho i've seen and heard it used enough for my own satisfaction.

Why? We say "i bhfeirg le" as well, and that's not a calque.

Sampla slán ó pheann an Chéitinnigh:
Beadhgais an bhean leis na briathraibh sin, ar mbeith dhi i ngrádh folaightheach ré Ceallachán ré cian d'aimsir roimhe sin, ón tráth do chonnairc i b-Port Lairge é, agus do-ní moichéirghe ar maidin ar n-a mhárach is téid ós íseal ar an raon 'n-ar shaoil Ceallachán do bheith ag teacht...
The woman started at those words, as she had been secretly in love with Ceallachán for a long time before that, from the time she saw him in Port Láirge, and she arose early the next morning and went quietly along the route on which she supposed Ceallachán would be coming...

(What words? and does she meet him in time? Read the whole exciting story here!)

Má tá "i ngrá le" maith a dhóthain don Ghaeilgeoir Mhór....

http://www.youtube.com/watch?v=0dOFOcKK0Lo


Tánn sibh cailcthe ag calque chuardach, a chairde!

quote:

could I say "táim lán grá..."? and the same question of what preposition to use...

"Táim lán de ghrá" duit/Dé/Mná 7 rl a bheadh agamsa.

tá mórán grá agam leatsa(?) duitse(SEA) ortsa(SEA)
ACH

Táim i ngrá doimhin[nó aidiacht eile] leat

Scríobh Pádraig:

quote:

I was once told by an Irish speaker that an expression such as gráim thú would be considered agressive and somewhat presumptious

Aontaíom leis sin. D'fheadfaí, freisin an chiall "I make love to you " a bhaint as.

Cad faoin leagan cainte "tugaim searc/grá duit"?

Cén cheist atá agat a DBnadT?

Tá an iliomad nathanna chun searc a chuir in iúl.

Is í an cheist cé acu a bhfuil an leagan cainte seo coitianta nó níl sé. Tá aithne agam ar an bhfocal "searc" de bharr an fhocail ghaolmhair Bhreatnaigh "serch", ach níor chualas i gcaint na ndaoine é riamh.

Maidir le "searc", is fíor duit. Faighim blas na litríochta, nó blas beagáinín ardnósach, air. Bhainfinn úsáid as i ndán grá, mar shampla. Dála an scéil, tháinig "grá" ó "grád" na Sean-Ghaeilge, ó "gradus" na Laidine, agus is é an chiall a bhí leis ar dtús ná "rang (= rank); onóir". Ach bhí "serc" sa teanga riamh leis an gciall "love".

Feicim sa FGB:

Mo ghrá thú is rud agat, I love you while you have something to give.


FRC-GRMA

Ceann deas - searbh.

Mo ghrá thú, a Dhaltai na Gaeilge, mar is rud agat,

Mo ghrá thú sibh , a Dhaltai na Gaeilge, mar is rud agat agaibh.

(is = agus anseo)

Ar scáth a chéile a fhoghlaimíonn na Daltaí!

Níl. Is aon duine amháin í...