PURE SPECULATION
(know your canon from your fanon)

• Because actor Glenn Quinn is from Dublin and emigrated to the United States with his mother in the 1980s, the character's backstory in fan fiction often parallels Quinn's. However, Doyle's nationality is never directly addressed in the series. Cordelia assumed that Doyle and Harry married in a "green card thing", however Harry's response does not indicate whether he is an Irish citizen with a green card, or is in fact an American citizen.
  
  
• Doyle's mother's whereabouts are a complete mystery. It is generally assumed by fans and in fan fiction that she is Irish, and that "Doyle" is her surname and she gave her surname to her son. However, there is no canonical reference one way or the other.
  
  
• Doyle is often portrayed as an hopeless alcoholic, however canonically he has had a beer (the 40oz beer, or "Billy Dee" he purchases after first confronting Angel in his apartment in City of...), three shots of whiskey (the bad poly-malt in I Fall to Pieces, and two shots of Angel's whiskey in Bachelor Party) and one hang-over after a night of drinking (the "Bam-Bam" incident, In The Dark). His persona of a drunken gambler is implied, rather than shown.
  
  
• Doyle's age is never stated, but he is assumed to be in his mid to late 20s. However, writers and fans often adjust his age to suit the story they wish to tell. However, he must be at least of 23 years old as Harry states in Bachelor Party "It was a 'madly in love couldn't live without each other' kind of thing. But I guess times change, because here we are, four years later and living just fine." Depending on interpretation, the 4 years can either mean since they wed (at age 19), or since they split (at age 21). Actor Glenn Quinn was born in 1970, and if Doyle were the same age as Quinn, that would have made him 29 years old during the first season of Angel.
  
  
• Despite Harry's line to Cordelia in Bachelor Party that Doyle already had his teaching credentials when they met, Doyle would not have gotten his leaving cert. at the age of 19, as this would have been pretty much impossible and that this was an error made on the part of the writers. Write around this as you will. However, the reality is that in order to teach in the public school system United States, he would have needed to have been certified by the Board of Education in whichever US city he was residing. However, Private Schools do not have the same criteria and usually require either a bachelors or masters in education.