Why do some people adopt an ethnicity that does not appear to be their own?

One answer obviously is that this can happen growing up within a community of a different dominant ethnicity.  Another is changing personal circumstances (such as marriage, emigration etc).  Other obvious reasons could include the prospect of personal gain, a need to "adopt a cause", or empathy with a group seen to be victimised.

To return to Rachel Dolezal, some aspects of her claims cast doubt on her veracity, and have caused the American public to largely believe that at least part of her motivation is self-interest.

Dolezal recently questioned whether the parents, who exposed her as a white woman, were really her mum and dad, saying “I haven’t had a DNA test. There’s been no biological proof that Larry and Ruthanne (Dolezal) are my biological parents”. 

An unusual family dynamic is that Dolezal obtained custody of her adopted brother Izaiah, who accused both his parents of physical abuse in a 2010 emancipation application (that was eventually dropped).  Another brother has disputed the allegations of abuse against their parents, believing Rachel made them up.  Rachel is also believed to have accused her ex-husband Kevin Moore of forcing her to take part in a homemade sex tape, in a bid to deny him access to their son Franklin.

Explaining Mac Stíofáin's claimed Irish ethnicity is more complicated.  His marriage to an Irishwoman and emigration to Ireland certainly would have affected his identity in later life, except that he seems to have been deeply involved in things Irish and militant republicanism at a much earlier stage.  

Mac Stíofáin said he always considered himself to be Irish, and in his teens joined the IRA.  "When I was very young," he recalled, "my mother had said to me, 'I'm Irish, therefore you're Irish. You're half Irish anyway. Don't forget it.'   I never did".  His adolescent enthusiasm for Irish republicanism thus seem to have been in part fostered by his upbringing and his mother's imagination.  Given that his mother died when he was only 10, his continued adoption of an Irish identity and support for militant republicanism nevertheless must have been in large part his own deliberate choice. 

Mac Stíofáin is not alone in being an English convert to militant Irish republicanism.  Another high profile convert to Irish causes was Rose Dugdale, who rebelled against her wealthy English upbringing and was a prominent IRA member.  While she remains in Ireland and is a veteran activist in Sinn Féin (following her release from prison), it is less clear whether she has totally changed her ethnic identification.

Overall, recognition of self-identity is a matter that is taken very seriously by some people, and non-acceptance can result in an emotionally charged reaction.