In 870 CE the stronghold of Alt Clut (now Dumbarton Rock) fell after a four month siege to a Viking army from Dublin, led by Amlaib and Imar, the ruler of Dublin and his brother, respectively. Alt Clut was the center of a small but wealthy Brittonic kingdom and controlled access to the lowlands of what is now Scotland. Possible maritime competition with Dublin may have played a part in motivating the siege, in addition to the prospect of loot. According to Irish chronicles, the Vikings returned to Dublin with a fleet of 200 ships, carrying not only captive Britons but also Angles and Picts captured in earlier raids.
Suppose instead that somehow Alt Clut was sufficiently forewarned of the coming attack and managed to annihilate the Viking force, capturing or killing Amlaib and Imar in the process. This severely weakens Dublin, the most powerful Viking settlement in Ireland, who might then be attacked by native Irish kingdoms that had suffered from frequent raids to feed the Dublin slave emporium, perhaps the largest in western Europe. If Dublin falls which Irish kingdom stands most to gain? Do other Viking settlements fall quickly in Ireland and Scotland? How does the slave trade shift in western Europe? Any thoughts are appreciated.