First of all I'd like to say thanks very much to Laighneach and Dennis for helping me the last time I wrote here. But the fact is that I've actually been thinking of getting a tattoo, so yeah, it's kind of permanent and thus I'd like the Irish to be absolutely correct. Now the problem is I'd like it to be kind of rhythmic, so it's also important that the syllables are the same (i.e. 4).

Here it is: A Béal an díon | Ma fís mo shaol | na múscail as | an bhfis mé 'nois | Beidh solas glé | na gealaí 'ris |

As you can see I've omitted the first syllable in "anois" and "aris"; is that totally unacceptable? Also, what literally does "A béal an díon" mean? I think it is a placename for a hill/look-ut point, but it's just a guess. Please translate the last two lines and also please correct me if you find any spelling or grammatical mistakes.
Is mise le meas/ Alasdair

Your going to need a good pain threshold for a tattoo that long!Anyway.....

Dennis, we don't want this poor young man scarred for life with bad irish grammar on his body, so make sure you question anything that I say, if it looks iffy.Or anyone else for that matter.

Firstly,"A Béal an díon" looks grammatically wrong, because if tranlated literally, it would come out as "Her mouth, the roof"(??????)I have no idea what it's meant to mean.
(Could it be vocative case - "O Mouth/Opening of the roof", which would be "A Bhéil an dín".
That could relate to the moon, in the last sentence.

The second one you have is "Má fís an saol", which looks fine, only that there should be an 's' at the end of the first word -"Más fís an saol".

"Ná múscail as an bhfís mé 'nois" is fine too, just remember the "síntí fada"(accents).I see no problem with dropping the first vowel- makes it sound more venacular, which is no bad thing.

Lastly "Beidh solas glé na gealaí arís" is grammatically correct, but has a word missing, as it translates as "The clear light of the moon will be again".I'm thinking that you want it to be "The clear light of the moon will be HERE/THERE again", which would be "Beidh solas glé na gealaí ann 'rís"

Once again Alasdair, I'm fairly confident of my grammar, but as they say, "Níl saoi gan locht", so make sure you get a second or even a third opinion on this board, before you go under the needle!

Couldn't the "A" in "A Béal an díon"/"A Bhéil an dín" in this case mean "to"?

"To" would be "Do" or "Ar son", not "A".If it really is "A", it could only be a possessive pronoun, either his/her/their, or the vocative case, in which you would be talking to or adressing the "Béal".