Irish orthography has evolved over many centuries, since Old Irish was first written down in the Latin alphabet in about the sixth century AD. Prior to that, Primitive Irish was written in Ogham. Irish spelling is mainly based on etymological considerations, very much like English orthography, although a spelling reform in the mid-20th century simplified the relationship between spelling and pronunciation somewhat.
There are three dialects of spoken Irish: Ulster (now predominantly in County Donegal), Connacht (Counties Mayo and Galway), and Munster (Counties Kerry, Cork, and Waterford). Some spelling conventions are common to all the dialects, while others vary from dialect to dialect. In addition, individual words may have in any given dialect a pronunciation that is not reflected by the spelling (rather like the English word colonel, whose spelling denotes its pronunciation quite poorly).

Consonants
The following chart indicates how written vowels are generally pronounced. Each dialect has certain divergences from this general scheme.

When e, é, i, or í come after or before a consonant, they make the consonant slender.
Between a consonant and a vowel, or vice-versa, e and i are usually silent, and just indicate that the adjacent consonants are slender. However, they may be pronounced in the digraphs ei, ia, io, oi, ui.
The accented letters é and í are always pronounced.
In digraphs and trigraphs containing a vowel with an acute accent (known in Irish as a fada or síneadh fada), only the accented vowel is normally pronounced. The epenthetic vowel
In verb forms some letters and letter combinations are pronounced differently from elsewhere.
In the imperfect, conditional, and imperative, -dh is pronounced /tʲ/ before a pronoun beginning with s-:
Otherwise it is pronounced /x/:
In the preterite impersonal, -dh is pronounced /w/:
-(a)idh and -(a)igh are pronounced /ə/ before a pronoun, otherwise /iː/:
In the future and conditional, f (broad or slender) has the following effects:

mholadh sé /ˈwɔɫ̪ətʲ ɕeː/ "he used to praise"
bheannódh sibh /ˈvʲan̪ˠoːtʲ ɕɪvʲ/ "you (pl.) would bless"
osclaíodh sí /ˈɔsˠkɫ̪iːtʲ ɕiː/ "let her open"
mholadh an buachaill /ˈwɔɫ̪əx ə ˈbˠuəxəlʲ/ "the boy used to praise"
bheannódh na cailíní /ˈvʲanoːx n̪ˠə ˈkalʲiːnʲiː/ "the girls would bless"
osclaíodh Siobhán /ˈɔsˠkɫ̪iːx ˈɕʊwaːn̪ˠ/ "let Siobhán open"
moladh é /ˈmˠɔɫ̪əw eː/ "he was praised"
beannaíodh na cailíní /ˈbʲan̪iːw nə ˈkalʲiːnʲiː/ "the girls were blessed"
molfaidh mé /ˈmˠɔɫ̪hə mʲeː/ "I will praise"
molfaidh Seán /ˈmˠɔɫ̪hiː ɕaːn/ "Seán will praise"
bheannaigh mé /ˈvʲan̪ˠə mʲeː/ "I blessed"
bheannaigh Seán /ˈvʲan̪ˠiː ɕaːn/ "Seán blessed"
After vowels and sonorants (/ɫ̪ lʲ mˠ mʲ n̪ˠ nʲ ɾˠ ɾʲ/) it is pronounced /h/:

• molfaidh /ˈmˠɔɫ̪hiː/ "will praise"
  dhófadh /ˈɣoːhəx/ "would burn"
  déarfaidh /ˈdʲeːɾˠhiː/ "will say"
  It makes a voiced obstruent (/bˠ bʲ vʲ d̪ˠ g/) or /w/ voiceless:
  
  
  
  • scuabfadh /ˈsˠkuəpəx/ "would sweep"
    goidfidh /ˈgɛtʲiː/ "will steal"
    leagfadh /ˈlʲakəx/ "would lay"
    scríobhfaidh /ˈɕcɾʲiːfˠiː/ "will write"
    shnámhfadh /ˈhn̪ˠaːfˠəx/ "would swim"
    It is silent after a voicless obstruent (/k c x ç pˠ pʲ sˠ ɕ t̪ˠ tʲ/)
    
    
    
    • brisfidh /ˈbʲɾʲɪɕiː/ "will break"
      ghlacfadh /ˈɣɫ̪akəx/ "would accept"
      But in the future and conditional impersonal f is often /fˠ, fʲ/
      
      
      
      • molfar /ˈmˠɔɫ̪fˠəɾˠ/ "one will praise"
        dhófaí /ˈɣoːfˠiː/ "one would burn"
        scuabfar /ˈsˠkuəbˠfˠəɾˠ/ "one will sweep"
        brisfear /ˈbʲɾʲɪɕfʲəɾˠ/ "one will break"
        In the past participle th (also t after d) is silent but makes a voiced obstruent voiceless:
        
        
        
        • scuabtha /ˈsˠkuəpˠə/ "swept"
          troidte /ˈt̪ˠɾˠɛtʲə/ "fought"
          ruaigthe /ˈɾˠuəcə/ "chased"