An adjective must agree with the noun it qualifies in gender, number and case.
1. masculine noun, nominative singular.
fear cliste, léachtóir maith, leabhar buí, ardán mór.
2 feminine noun nominative singular
bean cHliste, léachtlann mHór, iris bHeag, bróg cHlé.
3. Plural nouns.
daoine deasA, teagascóirí maithE, pinn gHormA, fillteáin bHeagA.
4. Nouns, genitive case.
hata an fHir mHóIr, cáilíochtaí na mná cliste.
A singular feminine noun in the nominative is always followed by an adjective with séimhiú, irrespective of the article, so - an bhó bHán, or - bó bHán.
A singular masculine noun in the nominitive is followed by an adjective with no séimhiú.
In the plural, a séimhiú is placed on an adjective which follows a noun ending in a narrow consonant.
Oh dear! I'm getting confused myself at this stage, I've just realized I've started a whole chapter. And the dreaded tuiseal ginideach(genitive) hasn't even been mentioned.
For examples of adjectives in all the different forms, go to
http://www.csis.ul.ie/focloir/ put the adjective in the box and enter.
Other on-line examples are available at -
http://www.ling.uni-potsdam.de/~green/aidiacht.htm the instruction is in German but you should be able to make out the examples in Irish.
Another good grammer resource is -
http://mbm.8m.com/irish/ Slán anois
Antóin