This is a rhyme taken from the book of the same name. When you say Inniu an Luan, does that mean that the verb 'is' is implied? ie does it mean Today is Monday or just Today Monday ?
The other question I have is: what's the nuance between ag bruíon (se bagarrer?) and ag achrann (se disputer?) ?

Inniu An Luan
Beimid ciúin

Inniu An Mháirt
Beimid i bpáirt

Inniu An Chéadaoin
Beimid ag bruíon

Inniu An Aoine
Beimid ag caoineadh

Inniu An Satharn
Beimid ag achrann

Inniu An Domhnach
Beimid ag foghlaim

Inniu an Luan

You can also say:
Is é an lá inniu an Luan.