Belfast City Hall was lit up red on Saturday evening to recognise the campaign for an Irish Language Act.
Unionists say they don't agree with a decision to light up the City Hall to support the campaign.
A small demonstration also took place in support of Irish language rights.
However Ulster Unionist councillor Chris McGimpsey said the City Hall should not be lit up for political issues and is questioning the council's decision to support the campaign in the first place.
"It's not what this is meant to be about. We've agreed that we would light up the city hall to support particular events like Pride week, Orangemen's day, St Patrick's day and so on," said Mr McGimpsey.
I don't know whose decision it was but they've decided to light up city hall in support of a Sinn Féin campaign for Irish language - it should not be about party political endeavours at all.
Speaking to UTV at the rally, Ciarán MacGiolla Bhéin from An Dream Dearg said: We approached Belfast City Council early last year and they put through a motion in support of our campaign.
"We then approached them about giving a physical demonstration that's important and they wholeheartedly supported that.
"What we are seeking is parity of esteem, what we are seeking is implementation of agreements that were made to our community.
We live in a diverse city, we live in a diverse society and the fact that Belfast City Council can recognise and celebrate that diversity though lighting these lights, it's something that brings us all closer together.
Mr MacGiolla Bhéin said when Stormont is back up and running, he wants Irish Language to be placed on top of the priority list.