I note you've got a citation point for "where the Irish community made up more than 10% of the population"; ledes should be either completely cited or not cited at all (excluding quotes).
I'm aware that there are differing views on citations in the lead, none of them supported by the GA criteria. I tend to prefer a citation-free lead myself as well though, so I've removed it. --MalleusFatuorum 22:07, 25 January 2010 (UTC)
I would suggest compressing the four short paras into two long ones.
I've compressed it into three. --MalleusFatuorum 22:07, 25 January 2010 (UTC)
"a group of 30–40" - "a group of 30–40 people"? I know they're unlikely to be confused for, say, ducks, but it helps clarify.
Changed that to 30–40 Fenians. --MalleusFatuorum 22:07, 25 January 2010 (UTC)
The lead seems to be in reverse order. You start by saying they were sentenced to death, then that two other sentences were overturned, then why they were sentenced to death. I'd suggest changing the order to have the para starting "Allen, Larkin, and O'Brien" before "The sentences of execution on".
It did look a bit muddled when I looked at it again, so I reorganised the whole thing. --MalleusFatuorum 22:07, 25 January 2010 (UTC)
"The whole of Ireland had been " - unnecessary, just "Ireland had been"; additionally, probably best to link Ireland to our article on the body of land.
I think that's a good idea, so I've done it. I'm less certain that it'll stick with the Irish Republican editors though, but we can but hope. --MalleusFatuorum 22:58, 25 January 2010 (UTC)
"The Irish Republican Brotherhood (IRB) was founded on 17 March 1859 by James Stephens, aimed at the establishment of an independent democratic republic in Ireland" - "was aimed", or alternately just "The Irish Republican Brotherhood (IRB) was founded on 17 March 1859 by James Stephens, and aimed for the establishment of an independent democratic republic in Ireland".
Changed to "with the aim of establishing ...". --MalleusFatuorum 01:01, 26 January 2010 (UTC)
"number of Fenians and Fenian sympathisers".. you need to make it clear that you're referring to the IRB membership as well, and 100k Americans didn't choose to move to the UK just to piss off some bastard Englishmen.
I don't agree that's what the sentence is suggesting. I think it's fine as it is. --MalleusFatuorum 00:57, 26 January 2010 (UTC)
"During the early hours of 11 September 1867, police arrested two men found loitering in Oak Street, Shudehill, suspecting them of planning to rob a shop. Both were charged under the Vagrancy Act and held in custody. The Manchester police were initially unaware of their identities, but their colleagues in Ireland identified them as Kelly and Deasy." - is unreferenced.
"The rescue inspired many to join the Irish cause for independence, and was also the inspiration for the song "God Save Ireland", Ireland's unofficial national anthem until it was replaced by "Amhrán na bhFiann" ("The Soldier's Song"). The events were important in shaping physical force Irish republicanism, the strand of Irish nationalism later represented by the Provisional Irish Republican Army. The events also – several years later – served to bring the parliamentary nationalists of the Irish Parliamentary Party under new leader Charles Stewart Parnell closer to the physical force men. Parnell told the Commons "there was no murder", and so helped create the conditions for the New Departure and the Irish National Land League and the subsequent "Land War" struggle against landlordism." - is unreferenced.