That's all very fine, but.... (said ironically) - ach tá sin go breá (but I'm not sure that that's very ironic, perhaps nach is needed here?)

“Gaelic Idioms” by G. Bannister has “Tá sin ceart go leor, ach….” for “That’s all very well but….” Not sure about the irony, though....

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Táim ag foghlaim Gaelainn na Mumhan

Tá fáilte roim nach aon cheartú!
I am a learner. Any translations offered are practice and should not be used unless confirmed.

Go raibh maith agat - that looks better than what I've come up with (I must get hold of that book one day)
Here's the context - (I'm still slowly and painfully working my way through the story of Féidlim Tonn Rí 's Castle as told by Seamas Ennis) -

"Ah," says the Raven,"that's all very fine but I'm now putting you under geasa to go travelling and not to sleep the second night in any one house or eat the second meal off any one table and to find Féidlim Tonn Rí's Castle within a year and a day or misfortune will befall you."