Civil War information

Your best bet is to check out the Bureau of Military History website, which has witness statements from former members of the IRA:
http://www.bureauofmilitaryhistory.ie/index.html

There's a search function on that. Statements you could look through include:
WS0458 Sean Corr
WS0711 John Travers
WS0721 Nicholas Smyth
WS0763 Seamus McCann
WS1004 Daniel Kelly
They were all members of 2nd Northern, there may be others from that division as well.

Some books worth reading on the north during that general period, though I don't think any of them go into any great depth on the Ronanstown attack:
Robert Lynch The Northern IRA and the Early Years of Partition 1920-1922
T. Ryle Dwyer Michael Collins and the Civil War
Liam O Duibhir Donegal and the Civil War (relevant cos anti-Treaty forces based in Donegal launched attacks across the border to coincide with those mounted by 2nd Northern)
Kieran Glennon From Pogrom to Civil War - Tom Glennon and the Belfast IRA (explores the real nature of the relationship between Collins / Mulcahy / O'Duffy on the one hand and the northern IRA on the other)

Probably not publicly known but the authorities would probably have had a fair idea of who was most likely to have been involved, going on the local knowledge of the B Specials in the area. Whether that "fair idea" would've been sufficient to secure a conviction in a court of law, I can't say - and whether your relative would've even been afforded the luxury of a trial, given how the Specials operated, is equally moot.

There's another book I should've referred you to in my earlier post - The Outrages, by Pearse Lawlor. On p270-273 he describes the initial ambush on the patrol of RUC and Specials at Ballyronan on 3rd May and the series of reprisal killings which followed, in Dungiven the next day, and in Ballyronan itself on 11th May, when three brothers were shot in their home by police, one fatally. Lawlor quotes from the Northern Constitution, a Derry local paper - the National Library have back issues on file. In that climate, it's not surprising that your relative made himself scarce.

As if all that weren't enough, bear in mind as well that internment was introduced in the north on 22nd May, which prompted a wholesale evacuation across the border by the 2nd Northern Division; a Free State civil servant's report "File on Pension Claims by Northern IRA" dating from approx 1924, said that by the end of June 1922, "There was only left inside the Division [2nd Northern] a small Flying Column of 6 or 7 Officers".

You mentioned that your relative died "soon after" going to the USA, but didn't specify the time period. If he survived after 1924 and made a pension claim for his service in the IRA, he may have given details of his involvement, actions in which he participated, etc. You should write to the Veterans Admin. Section, Dept. of Defence, Renmore, Galway - I think you have to be a direct descendant before they'll give you a copy of his file and even then, there's such a waiting list of queries that it'll be 9-12 months before you hear from them.