Thanks. And “ann”: yes, I just knew there was something definitely off with “aige”, there. I’d encountered “ann” before and had forgotten it. 
OK…here we go. But please keep in mind that I am far from perfect with Irish grammar or idiom, and don’t take any of this as gospel.
Your original was:
Inniu, bheimid go dtí teach tábhairne mar ag seinm ceol Sasanach. Níl fhios agam na cheol fós ach tá fhios a Máirtin é. Bheidh Máirtin a fhidil aige agus bheidh mé mo consairtín agam.
I would say:
Inniu, beimid ag dul (or “rachaimid”) go (or “chun”) teach tábhairne ceol Sasanach a seinm. Níl aithne agam ar an ceol, ach tá aithne ag Máirtín air [size=85][Note: This turned out to be incorrect. See my note below, and more in a message further down the thread]. [/size]Béarfaidh Máirtín a fhidil agus béarfaidh mé mo chonsairtín.
To break down the changes:
No lenition on “beimid.”
You need some form of the verb “to go” (téigh). You can either use “beimid ag dul” (we will be going) or “rachaimid” (we will go).
You only need the “dtí” in “go dtí” if you have the definite article. Otherwise, it’s just “go” (or you can also use “chun”). So you can say “go dtí an teach tábhairne” (to the pub) or “go (or “chun”) teach tábhairne” (to a pub).
I changed the way you’re saying “I don’t know” because I’m assuming that you mean that you’re not familiar with the music (but that Martin is). You use “fios” more for “information”…for example, if Martin knew exactly which tunes were going to be played, but you didn’t, you might use “fios.” I’m assuming that, in this case, it’s more a matter of you being less familiar with the English music than Martin is…if I’ve got that wrong, let me know.
[Edited to correct my mistake: Seems that “fios” IS correct in this context after all (a similar question came up on IGTF, so it gave me a chance to double check. So disregard what I said about “fios” vs. “aithne,” and see my post later on in this tread for more clarification]
It’s a minor point, because you don’t pronounce it when speaking, but if you were using “fios,” you’d want “níl a fhios agam/tá a fhios aige.” It really is easy to forget, however, because that “a” elides into the vowel sound of “fhios” when speaking.
There’s nothing technically wrong with saying “beidh Máirtín a fhidil aige agus beidh mé mo chonsairtín agam” (“Martin will have his fiddle and I will have my concertina.” Just remember to lenite after “mo”)…though I might word it “beidh a fhidil ag Máirtín agus beidh mo chonsairtín agam.” It just seemed to flow better to my ear to use a form of “beir”: Martin will bring his fiddle and I’ll bring my concertina.
Sounds like a great session! Surprisingly, we really don’t get much traditional English music here, and it would be fun to hear some!
Redwolf
Thanks! I am going to study this response in more detail later today. Interestingly I have a mix of stupid mistakes (I knew it should have been mo chonsairtín! And I meant to put in ag dul, lol) and new information. Very very useful stuff.
Re fios versus aithne, probably either would have worked. He knows the tunes and plays them often. I have heard lots of the tunes but not enough to play them myself, except the ones that I’ve picked up through his practicing, but I also didn’t know which tunes they would choose to play. 
He probably didn’t know exactly which tunes either but he was sort of session co-leader so had a lot of influence!
Avanutria,
I checked with some folks at IGTF, and it turns out that you were probably right to use “fios” in your message above. Here are some examples I was given:
I don’t know that song: Níl a fhios agam an t-amhrán sin.
I don’t recognise that song: Ní aithnaíonn mé an t-amhrán sin.
Do you know the air/tune for that song?: An bhfuil a fhios agat an ceol a theanns leis an amhrán sin?
The person who gave me that is a native Connacht speaker, and I changed the spelling a bit to conform to the standard spelling you’re probably more used to.
Given that, your original construction “níl a fhios agam an ceol” should be correct. Sorry for any confusion! I’ve made note of my mistake in my original reply as well.
I’ll let you know if I get any more updates on my question over there.
Well, and I just got an update. Further input, this time from an Irish teacher:
Níl an t-amhrán sin ar eolas agam: I don’t know that song
Ní heol dom an t-amhrán sin : I don’t know that song
Maybe: Níl an t-amhrán sin ar m’eolas : I don’t know/am not familiar with that song
Redwolf
Interesting. Níl a fhios agam an focail “eolas”. Is focail coimín é?
“Eolas” = “knowledge/information.” Tá sé measartha coitianta. Is fréamh é don focal “eolaíocht” (“science”).
Redwolf
Ó, tuigim anois. Go raibh maith agat!
I gcuntas Dé, ach tá an-dheacair dom an struchtúr habairt sin. Tá an Ghaeilge deacair go leor cheana féin!
My goodness, but that sentence structure is very difficult for me. Irish is hard enough already! 
Would that be roughly analogous to “That song is not in my knowledge” (looks like “not-is the song that to-the knowledge at-me” and going from that…)?
Yeah…that’s more or less what it says. Don’t you just love Irish idioms? 
Redwolf
Well, I don’t know if it’s faint, faint genetic memory let loose or what, but experience tells me that if I’ve had enough to drink, “That song is not in my knowledge” is definitely something that might come out of my mouth.
I suppose it’s not that different from “that song is not in my repertoire” but less fancy.
Éirigh leis.
Related but different…if I wanted to say “I didn’t know that” could I say “Níor d’aithin mé sin”?
I think I’d stick with “fios” for that one.
Redwolf
Reminds me of a t-shirt that a fellow came back fom Ireland with, that read (if I recall it correctly), “Níl a fhios agam; is é as Gaeilge é”. Funny.
Red, what’s the past tense of fios?
Nano, I think I need to add that shirt to my Cafepress shop!
“Fios” is a noun, actually, so it’s “tá” that needs to change tense:
Bhí a fhios agam: I knew
Beidh a fhios agam: I will know
Ní raibh a fhios agam: I didn’t know
Ní bheidh a fhios agam: I won’t know
Redwolf