Next on Page 11, the word sealaíocht.
D’éirigh le dream ar leith díobh, Muintir Chanannáin a shíolraigh ó Shéadna ua Chonaill, an réim a bhaint amach i dTír Chonaill agus a coinneáil ar feadh tamaill gur bhain Muintir Mhaoldoraidh, dream eile de shliocht Shéadna, sealaíocht díobh uirthi.
What I find especially interesting here is the word "sealaíocht" and how it is used in this particular sentence. Of course, we can have a look at the other parts of the sentence two.
The first part is not difficult: "a particular group of them" - who "they" are, is not of interest here, as we are only analyzing the sentence - "Canannán’s people, who were Séadna ua Conaill’s descendants" - it says "a shíolraigh ó Shéadna ua Conaill", i.e. "who descended from S. ua C., who were of his semen/seed/offspring" (síol) - "succeeded in taking the power in Tír Chonaill" - i.e. Tyrconnell, or Donegal. Note that for "power" (more commonly cumhacht), Séamus Ó Searcaigh uses here réim, which above all means "course, career", but also "power, sway, authority".
Then it becomes a little trickier. You would expect …an réim a bhaint amach i dTír Chonaill agus í a choinneáil ar feadh tamaill, i.e. to attain power and to keep it. But in Ulster, they still might use the possessive adjective instead of the "object pronoun + a + lenited verbal noun" structure, like this:
mo choinneáil = mé a choinneáil
do choinneáil = thú a choinneáil
a choinneáil = é a choinneáil
a coinneáil = í a choinneáil
ár gcoinneáil = sinn a choinneáil
bhur gcoinneáil = sibh a choinneáil
a gcoinneáil = iad a choinneáil.
The right-hand versions are more modern and less provincially Ulster, but the Ulster way to say it is not at all wrong or obsolete, and you will often see it even in newer literature by native Ulster authors.
But let’s get back to our original sentence:
…gur bhain Muintir Mhaoldoraidh, dream eile de shliocht Shéadna, sealaíocht amach uirthi.
"Until" is usually translated into Irish as go dtí go/gur, but even mere go/gur can have that sense. So, this clause means "until Maoldoradh’s people, another group of Séadna’s descendants" (sliocht means descendants, a person’s children, the same as clann) - "until they attained/achieved/reached sealaíocht on it." But what does sealaíocht mean?
According to Ó Dónaill, it means alternation, taking turns. I don’t know if you can say in English "they took their turn of it", but the idea is, that Maoldoradh’s people had their turn to rule Tyrconnell. - Yes, I know we don’t use the name "Tyrconnell" in English anymore. However, in Irish Tír Chonaill is very common as a name for the Irish-speaking parts of County Donegal.